Added density and group connection tests

[bdpedigo]

Continuation of #992

• [ ] Does this PR have a descriptive title that could go in our release notes?
• [ ] Does this PR add any new dependencies?
• [ ] Does this PR modify any existing APIs?
  • [ ] 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

Closes https://github.com/microsoft/graspologic/issues/570

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

Any other comments?