FEAT: Ensure that all Nash equilibrium methods are callable as a function in Python

[tturocy]

All Nash equilibrium methods should be callable as a function, in both C++ and Python.

• In Python, internally we have created class wrappers for each algorithm; these will be removed as they simply create unnecessary overhead.
• In C++, we will keep algorithm classes (at least for Gambit 16). However, each algorithm should be callable simply as a function without having to instantiate a class.

[tturocy]

This is now complete, with the signal exception of enumpoly_solve. This is something of a more thorny set of issues to unpick, and as such #376 picks this up with a more detailed discussion; for 16.1 at least we won't expose this method in pygambit.

In C++, Nash algorithms all now have interfaces with functions, so using the classes directly is no longer required. However, the command-line tool interfaces have not been migrated. As we progress and look at each method individually, we will refactor the C++ implementations accordingly.