[dartpy] Allow setting background color for OSG windows

[chhinze]

Problem
In DART, the background color of an dart::gui::osg::Viewer can be set in the constructor with a ::osg::Vec4 vector. This behaviour is also exposed in the python bindings, but the type ::osg::Vec4 is not defined. As far as I see, there is curently no possibility to set a background color.

Solution
Adding an __init__ function binding for dartpy.gui.osg.Viewer and dartpy.gui.osg.ImGuiViewer with an Eigen::Vector4f would allow to set the color either with a numpy array or even via a list with 4 entries.
I already wrote a solution for myself (see master...chhinze:fix/pyColorSupportOsgViewer) and would like to merge it, if desired.

Alternatives
There may be some OSG python binding projects, but I would prefer reducing the number of required Python packages.

[jslee02]

The solution sounds good to me.

Indeed, I'd prefer minimizing the third-party library APIs to the public DART APIs for both C++ and Python. In that sense, I'd add a new constructor of dart::gui::osg::Viewer taking Eigen::Vector4f (and setClearColor(const Eigen::Vector4f), and update the Python bindings accordingly.

There may be some OSG python binding projects, but I would prefer reducing the number of required Python packages.

Agreed for the same reason above.

[jslee02]

I believe this is resolved by #1393. Thanks for the fix @chhinze!

[chhinze]

Thank you for merging it.