VolMesh cell attributes cannot be serialized

[petrasvestartas]

Describe the bug

https://github.com/compas-dev/compas/blob/f85415822840eb6c7223698446c335cef06ce55e/src/compas/datastructures/volmesh/volmesh.py#L173C1-L173C78

I guess, "cell_data": {str(cell): attr for cell, attr in self._cell_data}, needs to be changed to self._cell_data

To Reproduce

from compas import json_dump
from compas.datastructures import VolMesh

volmesh : VolMesh = VolMesh.from_meshgrid(8,8,3.5,5,3,2)

for cell in volmesh.cells():
    volmesh.cell_attribute(cell, 'a', 0)
    
json_dump(volmesh, "volmesh.json") 

Expected behavior
Serialization should work.

Screenshots
None

Desktop (please complete the following information):

• OS: Ubuntu
• Python version 3.9.10
• Python package manager conda

Additional context
None