Allow 'promotion' to same order without exception.
Add bezier surface promote_u,v_to routines for multi-degree promotion.
Add bezier surface to_cubic_u,v routines to approximate surface as cubic.
Add bezier surface equiparametric maximum distance routines.
Refactor piecewise surface splitting to expose lower-level interface.
Fix bug in piecewise surface split_v
Add minimum/maximum degree method to piecewise surfaces.
Add adaptive to_cubic converter for piecewise surfaces

At present, the to_cubic_v test case crashes.  I have not been able to
trace this crash.  Taking the working test case and .swap_uv()'ing the
surface does not make it work.