Bernstein basis polynomials

(It took me a bit longer to look at this again than I'd wanted to, but here we are.)

Added a Bernstein polynomial basis to your power basis. Refactored the PPoly class a bit to accommodate for a different basis. There is still a some copy-paste left, so a bit more refactoring might be in order. 

It's not yet feature-complete, but it's already usable for interpolation with derivatives: https://github.com/EvgeniBurovski/piecewise_p has a first shot at replacing the `PiecewisePolynomial`. It's not feature-complete either, but the rest seems to be rather straightforward, and I'll go for it as there's time.