Export nwk tree

[drdna]

It would be great if the upgma function would allow exporting of a nwk tree for plotting with external tools (e.g ggtree).

[clarkevansteenderen]

Hi Mark, thanks very much for the suggestion! That's a great idea, and I will incorporate it into the package soon. Please let me know if you have any other ideas for improvement, or if you encounter any bugs along the way.