This type represents a B geometry SURFace.


A bsurface is a type of surface (PK_CLASS_bsurf is a subclass of
PK_CLASS_surf).

The bsurf is defined by data contained in a PK_BSURF_sf_t.
Piecewise and splinewise representations of bsurfs are also available:
See PK_BSURF_piecewise_sf_t and PK_BSURF_splinewise_sf_t