BUG: interpolate/_ppoly: minor cleanups according to review comments
ENH: interpolate: add extrapolate attribute to PPoly and BPoly
DOC: interpolate/_ppoly: update docstring
MAINT: ppoly: refactor evaluations

allocate memory, reshape x, y etc in PolyBase.
TST: add tests for higher order random polynomials
BUG: ppoly: set _y_extra_shape in conversions