Port models to uniformly use `obj`-based data

[adamkewley]

See opensim-org/opensim-models#180

• Remove all vtp references from resources/
• Remove all vtp references from docs/
• Go through any .zip files in docs/ and make sure no zip files contain models that refer to vtp files

[adamkewley]

This is held up until I figure out how to convert VTP files to OBJ without ballooning the size of the OBJ due to numeric conversion noise.

[adamkewley]

I've investigated this and concluded that, for backwards-compat reasons, it isn't worth it.

The way that legacy OpenSim models work is that they can define Meshes that depend on files that are defined in a "global geometry directory". OpenSim GUI etc. have had this feature for a while. However, it means that there are potentially models "in the wild" that are dependent on (e.g.) scapula_hat.vtp existing and being valid in the global geometry directory.

Because of this, I can't remove many/any existing entries in the global geometry directory without potentially bricking external model files. So they'll have to stay (although I can minimize them, reduce numeric noise, remove unecessary fields - they don't need to perfectly precisely reproduce the original mesh because they're decorative). This also means that converting the global entries to OBJ has no upside: I must redundantly keep the legacy VTPs in-tree until I (e.g.) come up with some sort of IO virtualization to shim the legacy files at runtime.