Add support for SKEL, OpenSim models and BioAmass dataset visualization

[MarilynKeller]

This pull request is a merge of the fork https://github.com/MarilynKeller/aitviewer-skel

It contains 3 new renderable classes:

• SKEL (A skin + skeleton body model based on SMPL with a more anatomically correct rigged)
• OSIM (load OpenSim sequences using NimblePhysics)
• Markers (to load and display C3D motion capture data, with the individual marker names)

And 4 new examples:

1. load_SKEL.py

Load the SKEL model, the pose can be edited interactively thanks to @jasminhacker (pull request)

2. load_osim.py

Load an OpenSim sequence, by default BSM with its individual markers:

3. load_markers.py

Load an AMASS sequence with the original mocap markers AMASS was created from

4. load_bioamass.py`

Load a sequence of the BioAmass dataset, i.e. the OpenSim sequence and the corresponding SMPL sequence from AMASS.