Implement multiplication between polynomials with real scalar values

Minterpy now supports the multiplication between instances of polynomials (of all concrete classes) by real scalar values. Due to the universal rule of polynomial-scalar multiplication, the implementation is done at the abstract base class "MultivariatePolynomialSingleABC" that will be carried by all concrete classes.

The implementation includes left-side, right-sided, and in-place multiplication.

Additionally:

• A verification function to check if a value is a real scalar value has been added to the code base.
• The test suite has been extended to included polynomial-scalar multiplication.

This MR should resolve Issue #139 (closed).