Add gambit - Rename 'hesslab-gambit' to just 'gambit'

[jlumpe]

I initially named this recipe hesslab-gambit to differentiate it from another package named gambit on PyPI. However the prefix hesslab- was really only something that applied in the earlier days of the project, and now that adoption of the tool seems to be taking off I'm afraid it will just be confusing.

There are no name conflicts with conda-forge or the base channel, so I would like to release future updates as just gambit. I left the hesslab-gambit recipe in place so that people's current workflows won't break.

I also replaced the summary field of the old recipe with a notice and a link to the new one (because it seems like the summary is the only one shown on the README.html page).

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