MAP: Catalog of standard games/game generators #394

## Overview

A nice feature of some packages and libraries - such as R - is that they provide built-in access to "standard" example datasets. We do have a (admittedly not well curated) set of examples which are mentioned in the documentation, and which are kept in contrib/games.

The proposal is that we should package these standard games at least in pygambit, and make it easy to load them, perhaps by a submodule that allows them to be loaded by name (e.g. pygambit.examples.load('myerson_poker')). In this collection it would be lovely to include all of the extensive and strategic games from prominent game theory textbooks or important articles.

In addition, it would be useful to develop standard codes for building certain example families/classes of games - for example, Kuhn poker with varying deck sizes.

These would be quite useful to users as examples or starting points. In addition, they would be excellent to integrate within the test suite - as they would be natural games to use for testing purposes. (In fact, we might consider that as far as the test suite goes, we should either use these "standard" games or, if we need bespoke games for specific tests, either to build them using the API calls or embed the game file representation in the test suite module, rather than having separate .efg/.nfg files.)

Some discussion should be had on what's the best way to organise the calling patterns for this, to accommodate both having standard games serialised explicitly (whether in .efg, .nfg., .agg, or other formats), and functions which generate games based on payoff or structural parameters.