Refactor polynomial-polynomial addition in the Newton basis.

Refactor polynomial-polynomial addition in the Newton basis.

Following the refactoring of the Grid and MultiIndexSet classes, the polynomial-polynomial addition in the Newton basis has been refactored. The procedure is as follows:

• match the dimension of the operands
• unionize the grid
• unionize the multi-index set
• compute the Lagrange coefficients of the operands at the unisolvent nodes
• sum them up
• transform to the Newton basis
• and finally create a new instance

The test suite has been updated to include polynomial-polynomial addition in the Newton basis which is delegated to the polynomial test module instead of the Newton polynomial specific test module.

This commit should resolve Issue #166 (closed) and part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).