Addition of new interface for limit surface evaluation

[barfowl]

This pull request includes the addition of a new evaluation interface that treats a subdivision mesh more like a piecewise parametric surface primitive.

The new interface was placed in namespace "Bfr" for "Base Face Representation" as all concepts and classes relate to individual faces of the base mesh. It uses many of the tools from the Far interface internally but presents them in a more simplified and scalable form.

The pull request is divided into four commits with the following additions:

• ./opensubdiv/bfr with the complete implementation
• ./tutorials/bfr with tutorials illustrating common use cases
• ./regression/bfr_evaluate confirming accuracy with Far::PatchTable
• ./documentation files and images for a "Bfr Overview" page

All changes have been thoroughly vetted with Pixar. Documentation is still undergoing improvement but the overview provided as part of these changes gives a far more thorough description than can be given here.

[davidgyu]

Filed as internal issues #OSD-377 #OSD-378 #OSD-379 #OSD-380

[davidgyu]

Thanks, Barry!