Add games to the catalog that are not EFG/NFG file format

[edwardchalstrey1]

This feature request is for...

PyGambit

Requested enahancement or feature

In #735 we added the games catalog, but currently only the EFG & NFG game files from contrib/games (moved to src/pygambit/catalog_game_files) are included as games in the catalog.

Update this such that contributed games with other supported filetypes e.g. .agg are also added to the catalog.

[tturocy]

This is part of a really chunky project, see #393. I've edited that issue to add an explicit mention of adding suitable examples to the catalog.