Introduce keyword-only arguments/parameters in several methods of the multivariate polynomial ABC

Several concrete implementatios related to polynomial evaluation, differentiation, and integration may have additional parameters that must be passed. Currently, the abstract base rigidly imposes what parameters can be passed to, e.g., __call__, diff(), and partial_diff(). If we, say, have different alternative implementations of how the differentiation can be computed specific to a concrete type, there is no easy way to access it.

By allowing keyword-only arguments in the signature of such methods, we can be more flexible in changin the behavior of method calls. I'd prefer to make these additional parameters as keyword-only to have the call explicit.