Introduce keyword-only arguments in several methods of multivariate polynomial ABC.

The signatures of several high-level public methods belonging to multivariate polynomial ABC are modified to allow for keyword-only arguments so the behavior of the undelrying functions can be modified while respecting the expected common behavior of the methods accross different concrete implementations.

The methods whose signature are modified are:

• __call__() of MultivariatePolynomialABC
• _eval() of MultivariatePolynomialABC
• partial_diff() (resp. _partial_diff()) of MultivariatePolynomialSingleABC
• diff() (resp. _diff()) of MultivariatePolynomialSingleABC
• integrate_over() (resp. (_integrate_over()) of MultivariatePolynomialSingleABC

This MR should resolve Issue #136 (closed).

Additional changes:

• For consistency, the private method _eval() of MultivariatePolynomialABC is now set to be a static method. The call to the method inside __call__() method is modified accordingly.
• The docstrings for the methods mentioned above are updated.

I'm assigning this MR to myself and because there is no change in the functionality of the code whatsoever, no review is required so this MR does not hold back upcoming MRs. As soon as the tests are passed, the merge may commence.