Refactor polynomial-polynomial addition in the canonical basis.

Following the refactoring of the Grid and MultiIndexSet classes, the polynomial-polynomial addition in the canonical basis has been refactored. The procedure is as follows: match the dimension of the operands, unionize the grid, unionize the multi-index set, sum up the coefficients according to the multi-index set, and create a new instance.

Additionally:

• The concrete implementation of the polynomial-polynomial subtraction in the canonical basis has been removed in favor of using the concrete implementation of addition operation and the __neg__() dunder method (i.e., subtraction is an addition between the first operand and the negation of the second operand).
• The test suite has been updated to include polynomial-polynomial addition in the canonical basis which is now delegated to the polynomial test module instead of canonical polynomial specific test module.
• The dimension expansion of a polynomial instance to either a target dimension or to a target polynomial instance is now supported by the expand_dim() method. The procedure also makes sure that the underlying user and internal domains are expanded accordingly. Exceptions will be raised if they are incompatible.
• Introduce __len__() method to compute the number of polynomials within an instance (note that a single polynomial instance may contain multiple coefficient sets).

This MR should resolve Issue #161 (closed) and part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).