Refactor polynomial-polynomial addition/subtraction in the Lagrange basis.

The implementation of general polynomial-polynomial addition/subtraction in the Lagrange basis has been removed to avoid unnecessary coupling with the Newton polynomial class and the corresponding transformation class. LagrangePolynomial would be the most bare polynomial basis in Minterpy.

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

The test suite has been updated to verify the behavior of the changes; any redundancies have been removed.

This commit should resolve Issue #170 (closed) and part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).