Add polynomial basis transformation methods to Interpolant class

Several methods to transform the underlying Newton polynomial of an Interpolant instance to various polynomial bases are introduced. This includes:

• to_newton(): the interpolant as a polynomial in the Newton basis
• to_lagrange(): the interpolant as a polynomial in the Lagrange basis
• to_canonical(): the interpolant as a polynomial in the canonical basis
• to_chebyshev(): the interpolant as a polynomial in the Chebyshev basis

Note that, the method to_newton() is available to directly retrieve the Newton polynomial; it does not require any transformation because the instance already stores the interpolating polynomial in the Newton basis.

The corresponding test suite has been updated to validate these methods. Finally, the in-depth tutorials have been updated with reference to the new methods.

This MR should resolve Issue #182 (closed).