Refactor polynomial-polynomial multiplication in the Lagrange basis.

The implementation of general polynomial-polynomial multiplication in the Lagrange basis has been removed to avoid unnecessary coupling with the Newton polynomial class and the corresponding transformation. LagrangePolynomial would become the most bare polynomial basis in Minterpy (as long as the monomials are not supported "natively").

The support for scalar and constant polynomial multiplication remains because it does not require any transformation.

The test suite has been updated to verify the behavior of the changes.

This commit should resolve Issue #169 (closed) and part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).