Added density test and group connection test to inference module

[jwellan1]

• [X] Does this PR have a descriptive title that could go in our release notes?
• [X] Does this PR add any new dependencies?
• [X] Does this PR modify any existing APIs?
  • [X] Is the change to the API backwards compatible?
• [ ] Have you built the documentation (reference and/or tutorial) and verified the generated documentation is appropriate?

Reference Issues/PRs

Continuation of PR #981 (now using a new branch because prior branch was compromised)

What does this implement/fix? Briefly explain your changes.

This PR adds the density test and group connection test to the inference module, enabling statistical comparison of two networks under either the Erdos-Renyi model (density test) or the Stochastic Block Model (group connection test).

Any other comments?

[bdpedigo]

Review TODOs (for me)

• [x] review docstrings
• [x] fix typing issue
• [ ] review tests
• [ ] review notebooks
• [ ] benchmark against old code and simulations