Adds structured independent edge model

[ebridge2]

• [x] 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

Adds #929

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

Implements the SIEM model, for both estimation and simulation, as well as a utility to generate the edge cluster matrix from an SBM. This has the effect of allowing one to test community membership differences for the clusters of an SIEM or the blocks of an SBM.

Any other comments?

[ghost]

All CLA requirements met.

[bdpedigo]

closing since noone has reviewed, happy to revisit if someone can