DOC: fix bpoly docstrings
API: remove axis & direction arguments of bpoly.from_derivatives

(They were not implemented anyway.) The idea is to keep the basic
object simple, and delegate complications to subclasses or wrappers.
ENH: bpoly: implement in-place derivatives
DEP: Deprecate PiecewisePolynomial
Merge pull request #3320 from ev-br/poly_

ENH: interpolate: improve BPoly API and functionality

- remove unneeded attributes (axis and direction).
- implement in-place evaluation of derivatives.
- add tests for complex coefficients.
- deprecate PiecewisePolynomial