[FEA] Expose gamma (resolution) parameter for Louvain.

[jwyles]

This is to expose the gamma parameter (in the formula for modularity score) as a parameter the user can specify.

Since currently Louvain assumes a value of 1 for gamma some of the code will need to be rewritten in order to use a different value of gamma.

Modularity and the Louvain implementation are known to suffer from a resolution issues. To address that, iGraph has an option to set a resolution limit. That same feature should be added to our version.

https://scanpy.readthedocs.io/en/stable/api/scanpy.tl.louvain.html

Modularity is defined as
image

This modification is to allow the resolution parameter, y, to be specified in the range of 0.0001 to 1.0