Polynomial differentiation

This merge includes the interface and (some) implementation for the polynomial differentiation feature. Mainly it adds the following two interfaces to `MultivariatePolynomialABC':

• partial_diff(dim: int, order: int) -> MultivariatePolynomialABC Computes first order partial derivative along the dimension specified by dim. Returns a new polynomial instance.
• diff(order: np.ndarray) -> MultivariatePolynomialABC Computes a derivative of specific order along each dimension. eg. order=[2,3,1] will compute respectively 2nd order, 3rd order, and 1st order along the three spatial dimensions. Returns a new polynomial instance.

Concrete implementations are provided for CanonicalPolynomial (from Jannik's old code) and NewtonPolynomial.

This MR addresses issue #5 (closed).