USD Import: Add support for all UsdGeomSubset family names

[Vochsel]

UsdGeomSubsets, are USD's answer to multiple mesh assignments per single UsdGeomMesh.

Blender import and export currently authors:

s@familyName = "materialBind";
s@elementType = "face";

But we should be able to ingest all variations:

https://graphics.pixar.com/usd/docs/api/class_usd_geom_subset.html#a8ed09ac929b02f6225eacb6b033d3a49

• UsdGeomTokens->partition: implies that every element of the whole geometry appears exactly once in only one of the subsets belonging to the family.
• UsdGeomTokens->nonOverlapping: an element that appears in one subset may not appear in any other subset belonging to the family.
• UsdGeomTokens->unrestricted: implies that there are no restrictions w.r.t. the membership of elements in the subsets. They could be overlapping and the union of all subsets in the family may not represent the whole.