Fix/py color support osg viewer

[chhinze]

Fixes issue #1393. Allows to set the background color in dartpy.gui.osg.Viewer and dartpy.gui.osg.ImGuiViewer with a type convertible to an Eigen::Vector4d, e.g.

# set background to white
viewer = dartpy.gui.osg.Viewer(np.array([1,1,1,1]) 
# or even by using python lists, that get implicitly converted
viewer2 = dartpy.gui.osg.Viewer([1,1,1,1])

Should we add the new possibility to one of the dartpy examples?

The argument type is now Eigen::Vector4d(instead of Eigen::Vector4f), because it allows passing a list as well (as in the creation of viewer2above).

---

Before creating a pull request

• document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1398 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1398      +/-   ##
==========================================
- Coverage   57.38%   57.37%   -0.02%     
==========================================
  Files         366      366              
  Lines       27405    27405              
==========================================
- Hits        15726    15723       -3     
- Misses      11679    11682       +3

Impacted Files	Coverage Δ
dart/dynamics/EulerJoint.cpp	69.32% <0%> (-3.69%)	⬇️
dart/dynamics/Skeleton.cpp	66.22% <0%> (+0.16%)	⬆️

[jslee02]

Should we add the new possibility to one of the dartpy examples?

Yeah, it'd be worth to test it in at least one dartpy example (maybe hello_world_gui).

[jslee02]

LGTM!