AbcExport patch from Daniel Rolinek on behalf of Tippett.
1. only create face sets for shading groups applied with with per-face mappings
2. no longer produce an error for meshes without valid per-face mappings, e.g. whole object mappings
3. recognize "faceVaryingInterpolateBoundary",  not "faceVaryingInterpolateBoundary" as the control attribute for the related alembic property.
Merge pull request #6 from lamiller0/requestB

AbcExport patch from Daniel Rolinek on behalf of Tippett.