Implement SBM minimization with frozen vertices

Ni! This issue is intended to track work implementing vertex freezing during [h]SBM minimization, so that @count0 can see if I'm doing something stupid incompatible with the bigger picture.

(A link will be added to the relevant branch as soon as there is one.)

Approach

1. Make it work with a wrapper function around minimize[_nested]_blockmodel_dl().
2. User provides a frozen_vertices argument that will be translated to mcmc_args and shrink_args.
3. Returns a [Nested]BlockState that does not move frozen vertices and preserves the blocks [hierarchy] containing them.

I prefer frozen_vertices to vertices here, as it makes it easier for the user to grasp which blocks will be preserved. But I'd be fine with either.

Tasks

Make it work with blockmodels

• In minimize.py: min_B and max_B produced by get_states() should take into account frozen vertices.
• This will be done in the wrapper, so as to keep changes to the existing code to a minimum.
• Alternatively, one may be expected to provide appropriate b_min e b_max when calling.
• Implementing vertex_freezing for merge_sweep().
• Unlike its buddy mcmc_sweep(), merge_sweep() called in shrink() has no notion of a list of block vertices (not) to be modified.
• This will require changes to the existing code, including a new parameter frozen_vertices in shrink_args.
• Once this is done we can check if the later mcmc_sweep side of things in mcmc_equilibrate() justworks®.

Make it work with nested blockmodels

• Nada; depends on the previous.