Dating dangling nodes

[awohns]

We currently can't date an unsimplified tree sequence with dangling nodes. We might want to think about how to do this, the problem is that there isn't a clear prior for these nodes.

[hyanwong]

To be clear, we might want to leave dangling edges in place because they are useful attachment points for augmented samples. Unfortunately they may be hard (impossible?) to date because of the lack of prior

[hyanwong]

It should be possible to assign the global prior to these nodes now

[hyanwong]

Not longer an issue for variational_gamma.