Add recipe for PyPI package cell-gears

[jdblischak]

https://pypi.org/project/cell-gears/
https://github.com/snap-stanford/GEARS

• No one has submitted to staged-recipes or bioconda-recipes yet.
• All of its dependencies are available from conda-forge, so it is a judgement call where to add it. I decided on bioconda because 1) it's a single-cell method, and 2) it has a publication which can be added to the bioconda recipe metadata. Happy to migrate my PR to conda-forge if that would be preferred
• The dependencies are actually a bit complex. The first step in their installation instructions is to install PyG, but this isn't an explicit dependency. Turns out that PyG isn't available from either conda-forge or bioconda, but instead from its own channel: pyg::pyg

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