Fix Angle.__abs__ such that it always returns a positive value

[erogers-dstl]

Address part of issue #749 that deals with Bearings not playing nicely with numpy.isclose, which is due to abs(Bearing(-pi)) returning a negative value.

[erogers-dstl]

Note that (as per the comment in the source file), there is still the oddity that abs(Bearing(-pi)) != Bearing(+pi)) or equivalently that abs(Bearing(-pi)) != Bearing(abs(-pi)))

This is because abs(Bearing(-pi)) == pi (because abs forces it positive) and Bearing(abs(-pi)) == -pi (because the bearing modulo operation pushes pi to be stored as -pi).

[edwheelhouse-dstl]

Fine. The oddity is annoying, but hopefully it won't come up often!