Split sparse_tools.py into multiple files

[kevinsung]

I think there could be three files: one about states (get_ground_state, jw_configuration_state, etc.) one about operations (expectation, variance, etc.), and one about restricting to subspaces (jw_get_number_indices, etc.). Perhaps it deserves its own folder.

This would be a breaking change if we move it into a separate module.

[babbush]

Hmm. I don't think this is very high priority but it would probably make sense when we eventually reorganize modules.