Load OpenSpiel games via Gambit Catalog

[edwardchalstrey1]

Issues closed by this PR

Described in #760 but this only does OpenSpiel and only games that currently work

Description of the changes in this PR

• This PR is an alternative to Add games from OpenSpiel to catalog #878 that allows direct loading of OpenSpiel games compatible with (can be exported to) Gambit
• Allows the user to do gbt.catalog.load_openspiel("tiny_hanabi")
• Updates the documentation including OpenSpiel tutorial to load games via the catalog
• Uses mocking to test the OpenSpiel loading - users are not expected to have this installed
• Crucially, this code shouldn't need to be changed when more OpenSpiel games become exportable to NFG or EFG

How to review this PR

• Review `load_openspiel and testing strategy
• Review explanations in catalog doc page and OpenSpiel tutorial

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[edwardchalstrey1]

Thanks for your comments @tturocy was an easy refactor have a separate load_openspiel function to call - you can now pass the params in there