Geometry provider.

Start moving away from smtk::model::Tessellation in favor of smtk::resource::GeometryProvider, which allows zero-copy transfer of geometry from modeling kernels to rendering backends such as VTK. See doc/userguide/resource/geometry.rst for a description.

Example implementations are in TestGeometryProvider (all in 1 file) and smtk::session::polygon (spread across all operations, the custom VTK geometry provider and more).