Enhancement - Add PyBullet Solver Support

[VallesMarinerisExplorer]

Hi A2Plus community,

I have been using this library for a while and I think it is pretty good! I think that it could help FreeCAD become more accepted in the engineering community. Along these lines, I have started working on integrating PyBullet into A2Plus. Basically, whenever an axial joint is added, a new URDF is created/appended. URDF files can be used in frameworks such as PyBullet for physics/robotics modeling or in ROS for example. Take a look if you'd like and let me know what you think! Right now this only supports axial joints so will need to add more joint types and also re-implementing lining up the PyBullet joints in FreeCAD is proving to be tricky. Thanks and have a great day!

https://github.com/VallesMarinerisExplorer/A2plus/tree/master