Fix FarStencilTablesFactory: replace limit tangents table

The new table accomodates verts with valence up to 20.

Dr. DeRose provided me with a python script to generate the tables, but wants to
investigate a closed-form alternative to pre-computed tables.

Also: our logic is incorrect - it is missing adjacent face-points in the
interpolation (apparently this has been incorrect since day 1). Further fixes
coming next year...

fixes #246