Implement Hermite/Lagrange/Legendre polynomial lifting functions

[sdahdah]

Refactor PolynomialLiftingFn to support products of other polynomials instead
of just monomials. For example, instead of x1^2 * x2, allow H2(x1) * H1(x2)
where Hx is the xth Hermite polynomial.

This can be achieved by removing the wrapped scikit-learn polynomial
transformer and using a custom one with similar functionality. Specifically the
powers_ matrix.