Add method to check if the domain of two polynomials match

A new method has_matching_domain() is added to all polynomial instances. This method checks if a given instance has matching internal and user domains with another instance, irrespective of the basis.

This MR should resolve Issue #143 (closed).

Additionally:

• Originally, this functionality is provided by a private function inside the canonical polynomial module but also used in the lagrange polynomial module. Now as a method, it is available to all polynomial instances.
• Add the relevant tests to the test suite.