Implement Hermite/Lagrange/Legendre polynomial lifting functions

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 `x`th 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.