Refactor Polynomial-Polynomial Addition/Subtraction in the Lagrange Basis

Following Issue #169 (closed), we need to rethink whether LagrangePolynomial should support general polynomial-polynomial addition/subraction. Removing the support would minimize the level of two-way coupling between polynomial classes (and their corresponding transformation classes). Polynomials in the Newton basis following recent changes are fully capable of doing most of numerical tasks involving polynomials (and the transformation from Lagrange to Newton must be done intentionally).

The addition/subtraction of polynomials in the Lagrange basis with a scalar or a constant polynomial may be kept as it does not involve any (hidden) transformation.

This issue is part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).