Implement polynomial addition in the Chebyshev basis.

Implement polynomial addition in the Chebyshev basis.

Addition (and subtraction) between polynomials in the Chebyshev basis is now supported by a concrete implementation.

Additionally:

• Methods related to polynomial-polynomial and polynomial-scalar addition/subtraction in the abstract base class has been refactored to standardize their interface.
• Functions related to quadrature and construction of product polynomial and summed polynomial have been moved to the respective modules so they are located closer to where these functions are used.
• Additional tests have been added to the test suite to validate the new functionality.

This MR should resolve Issue #164 (closed) part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).