Pastml should stop with an error when internal node labels are not unique

[stitam]

It took me a while to discover that the reason I was getting funky pastml results was that my input tree had redundant internal node labels. While this is clearly a problem with my input data, I would have found it sooner if pastml had given me an informative error saying that I have non-unique internal node labels in my tree. Can such an input validation be added to pastml? FYI, I tested this with pastml v1.9.34, apologies if the issue has been fixed (commit messages did not suggest the issue has been fixed). Thanks, and also many thanks for developing pastml!

[stitam]

Adding linked issues:

ms609/TreeTools#149

emmanuelparadis/ape#113