From Nicholas Yue:  Boost_PYTHON_LIBRARY variable is case sensitive
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.
Revamp AbcStitcher to better handle cases where certain shapes or properties
may not exist in some of the files.

We will loosen the requirement of the hierarchy being the same across all
samples, or that there shouldn't be any gaps in time between archives.

Instead default values (or the previous value) will be set for scalar properties
for those missing samples, and for array properties, empty arrays will be set.
(or the previous sample in certain special cases)