DOC: bpoly: expand docstrings a bit.
ENH: bpoly: construct a polynomial from a list of derivatives
ENH: bpoly: raise a degree of a polynomial
EHN: BPoly: add a constructor from a list of derivatives at breakpoints
ENH: interpolate/_ppoly: change PPoly.c shape convention

Don't ravel the trailing dimensions of the coefficient array.  Removes
the need to inherit from _Interpolate1D.
MAINT: interpolate/BPoly: move helper routines to @staticmethods in BPoly