Add support for addition/subtraction of polynomials in the Newton basis

Addition and subtraction between polynomials and polynomials as well as between polynomials and real scalar numbers are now supported for polynomials in the Newton basis via __add__() and __sub__() dunder methods. Right-sided addition and subtraction are supported for real scalar numbers as the left operand via __radd__() and __rsub__() dunder methods.

Additionally:

• A static abstract method _iadd() is now added to the MultivariatePolynomialSingleABC as a placeholder for a concrete implementation of augmented addition (and subtraction). Currently, no concrete polynomial classes implement this method and augmented addition/subtraction will raise an exception.
• The test suite has been extended to test the behavior of polynomial-polynomial addition/subtraction and polynomial-scalar addition/subtraction in the Newton basis.

This MR should resolve Issue #140 (closed).