USD IO: fix compiler warnings and errors

Fixed warnings and errors for linux and darwin
builds. Also fixed copyright date in
usd_light_convert.cc.

USD IO: initial commit of extended features

Instancing import: Import USD scene instances as Blender collection instances.

Instancing export: Extend the existing instancing option to support exporting arbitrary object hierarchies as USD scene instances. Additional support for exporting Blender particle systems as USD point instancers.

Environment map IO: Logic to convert between USD dome lights and Blender world materials, including environment textures.

Unit conversion scene scale: Automatically scale the scene for unit conversion on import and export (e.g., scale the imported objects based on the USD?s meters per unit value).

Curve export.

Armature export: Export armatures and skinned meshes to USD skeletons and skeletal animations.

Light unit conversion: Experimental code to convert between light intensity units in Nits and Blender?s light energy units, on import and export.

Transform operator options: Option to save transforms to USD as the combination of scale, rotate and translate operators, where the rotation can be expressed as Euler angles or a quaternion.

Export to USD shader nodes: Convert Blender shader nodes to UsdPreviewSurface nodes, MDL material nodes or a custom USD representation of Cycles shaders. (MDL export requires UMM addon to be installed.)

Import MDL materials: Convert MDL materials to Blender shader networks. (Requires UMM addon to be installed.)

Texture export: An option to save textures to a directory relative to the USD being exported, using either absolute or relative asset paths. This feature works with UDIM tiles as well as packed and in-memory ?baked? textures.

Option to specify a default primitive on export.

Option to add a root primitive on export. This option adds a single prim as the parent of all exported prims.