Deprecate MassMigration class

[molpopgen]

Using these classes to define "bulk" migration events is a major headache, causing all sorts of special case logic in the code (both Python and C++). The way that we do things when importing demes models, which is to vary the ancestry proportions via migration rates, is much simpler and is the way forward.

[molpopgen]

We are on the way to rewriting the back end to
be based on demes graphs.
This issue will handle itself as a result.

[molpopgen]

Closed via #1064