Add support for solid angles mostly based on what we do for angles.

[adamreichold]

Closes #190

[iliekturtles]

Thanks for the PR! I'll do a code review today!

[iliekturtles]

Changes look good! I only found a couple formatting issues. If you can amend these into your commit that adds SolidAngle I'll merge!

diff --git a/src/si/mod.rs b/src/si/mod.rs
index eba8ae4..3700cf4 100644
--- a/src/si/mod.rs
+++ b/src/si/mod.rs
@@ -118,7 +118,7 @@ storage_types! {
 pub mod marker {
     use si::{Dimension, Quantity, Units};
     use Kind;
-
+    
     /// `AngleKind` is a `Kind` for separating angular quantities from their identically dimensioned
     /// non-angular quantity counterparts. Conversions to and from `AngleKind` quantities are
     /// supported through implementations of the `From` trait.
@@ -133,8 +133,8 @@ pub mod marker {
     pub trait AngleKind: ::Kind {}
 
     /// `SolidAngleKind` is a `Kind` for separating quantities of solid angles from other
-    /// identically dimensioned quantities. Conversions to and from `SolidAngleKind` quantities
-    /// are supported through implementations of the `From` trait.
+    /// identically dimensioned quantities. Conversions to and from `SolidAngleKind` quantities are
+    /// supported through implementations of the `From` trait.
     ///
     #[cfg_attr(feature = "f32", doc = " ```rust")]
     #[cfg_attr(not(feature = "f32"), doc = " ```rust,ignore")]
@@ -145,9 +145,9 @@ pub mod marker {
     /// ```
     pub trait SolidAngleKind: ::Kind {}
 
-    /// InformationKind is a `Kind` for separating information quantities from their identically
-    /// dimensioned non-information quantity counterparts. Conversions to and from
-    /// `InformationKind` quantities are supported through implementations of the `From` trait.
+    /// `InformationKind` is a `Kind` for separating information quantities from their identically
+    /// dimensioned non-information quantity counterparts. Conversions to and from `InformationKind`
+    /// quantities are supported through implementations of the `From` trait.
     ///
     #[cfg_attr(feature = "f32", doc = " ```rust")]
     #[cfg_attr(not(feature = "f32"), doc = " ```rust,ignore")]
diff --git a/src/si/solid_angle.rs b/src/si/solid_angle.rs
index b134441..95773f2 100644
--- a/src/si/solid_angle.rs
+++ b/src/si/solid_angle.rs
@@ -14,8 +14,8 @@ quantity! {
         Z0>;    // luminous intensity
     kind: dyn (::si::marker::SolidAngleKind);
     units {
-        /// SI derived unit of solid angle is steradians. It is the solid angle
-        /// subtended at the center of a unit sphere by a unit area on its surface.
+        /// SI derived unit of solid angle is steradians. It is the solid angle subtended at the
+        /// center of a unit sphere by a unit area on its surface.
         @steradian: 1.0_E0; "sr", "steradian", "steradians";
         @spat: 1.256_637_061_435_917_3_E1; "sp", "spat", "spats";
         @square_degree: 3.046_174_197_867_086_E-4; "°²", "square degree", "square degrees";
@@ -26,7 +26,8 @@ quantity! {
 
 #[cfg(feature = "f32")]
 impl SolidAngle<::si::SI<f32>, f32> {
-    /// The solid angle subtended by a sphere at its center, i.e. with a value 4π as measured in steradians
+    /// The solid angle subtended by a sphere at its center, i.e. with a value 4π as measured in
+    /// steradians.
     pub const SPHERE: Self = Self {
         dimension: ::lib::marker::PhantomData,
         units: ::lib::marker::PhantomData,
@@ -36,7 +37,8 @@ impl SolidAngle<::si::SI<f32>, f32> {
 
 #[cfg(feature = "f64")]
 impl SolidAngle<::si::SI<f64>, f64> {
-    /// The solid angle subtended by a sphere at its center, i.e. with a value 4π as measured in steradians
+    /// The solid angle subtended by a sphere at its center, i.e. with a value 4π as measured in
+    /// steradians.
     pub const SPHERE: Self = Self {
         dimension: ::lib::marker::PhantomData,
         units: ::lib::marker::PhantomData,
@@ -97,11 +99,15 @@ mod tests {
         fn check_units() {
             Test::assert_eq(&SolidAngle::new::<sa::steradian>(V::from_f64(4.0 * PI).unwrap()),
                 &SolidAngle::new::<sa::spat>(V::one()));
-            Test::assert_approx_eq(&SolidAngle::new::<sa::square_degree>(V::from_f64(360.0 * 360.0 / PI).unwrap()),
+            Test::assert_eq(
+                &SolidAngle::new::<sa::square_degree>(V::from_f64(360.0 * 360.0 / PI).unwrap()),
                 &SolidAngle::new::<sa::spat>(V::one()));
-            Test::assert_approx_eq(&SolidAngle::new::<sa::square_minute>(V::from_f64(60.0 * 60.0).unwrap()),
+            Test::assert_eq(
+                &SolidAngle::new::<sa::square_minute>(V::from_f64(60.0 * 60.0).unwrap()),
                 &SolidAngle::new::<sa::square_degree>(V::one()));
-            Test::assert_approx_eq(&SolidAngle::new::<sa::square_second>(V::from_f64((60.0 * 60.0) * (60.0 * 60.0)).unwrap()),
+            Test::assert_eq(
+                &SolidAngle::new::<sa::square_second>(
+                    V::from_f64((60.0 * 60.0) * (60.0 * 60.0)).unwrap()),
                 &SolidAngle::new::<sa::square_degree>(V::one()));
         }
     }

[adamreichold]

@iliekturtles Applied the patch. Could you give me a hint on how you keep the formatting in check as rustfmt will not touch the stuff inside the macros reliably? (I have admittedly become dependent on rustfmt and hardly even consider formatting while writing code anymore.)

[iliekturtles]

Build failed which is my fault! I was testing assert_approx_eq vs assert_eq and forgot to remove that before posting the patch. If you can switch the last three back to assert_approx_eq everything should work then I can merge for real.

For formatting I just spend a bunch of time looking at the code. I have vertical guidelines setup at the 100 character mark to see when text overflows and VIM's gq operator mostly works to re-wrap comments.

[adamreichold]

Switched back to assert_approx_eq.

[iliekturtles]

Thanks again for the PR. Merged!