Refactor the multiplication of polynomials in the Newton basis.

The multiplication of polynomials in the Newton basis now employs the multiplication of respective grids and multi-index sets. Furthermore, it can handle the multiplication of polynomials whose indices are separate (the multi-index set of the polynomial differs from the set of the grid of the polynomial).

Additionally:

• The test suite has been updated to verify the new changes.
• The identity comparison for the 'multi_index' attribute in all polynomial instances has been replaced with equality comparison to ensure the comparison is based on the actual content of the multi-index sets not their id.

This commit should resolve Issue #165 (closed) part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).