Add directed SBM parameter estimation

[ragibson]

The SBM equivalence used for gamma estimates assumes drawing from an undirected stochastic block model even when the network is truly directed.

For our work, this only affects the Lazega Law Network experiment where we were comparing our method's performance to that of Roxana et al.'s. They made the same decision to use the undirected form for the gamma estimates, so the apparent mismatch is appropriate.

That said, we should add in the ability to use the directed form of the gamma estimates (seemingly only requiring a tweak of sum(k_c^2) to sum(k_{c,in} * k_{c,out}) in the calculation) which would restore the one-to-one correspondence between modularity hyperplanes and parameter estimates in the directed (single-layer) case.