Fix serialization of VolMesh cell attributes

[ArianMoeini]

Summary

• Serialize VolMesh cell attributes from key-value pairs so cells with custom attributes can be saved.
• Add a JSON round-trip regression test with attributes on multiple cells.
• Add contributor credit as requested by the contribution guide.

Closes #1396.

Tests

• PYTHONPATH=src:/private/tmp/compas-test-deps python -m pytest tests/compas/datastructures/test_volmesh.py -q — 37 passed.
• git diff --check — passed.