avoid gui lockups when calculating trees & pca for large alignments

Noticed when looking at JAL-4078 - building a tree (on OSX with FlatLAF) from a >40000 sequence alignment seems to hog the gui thread. Ideally the tree panel should appear relatively quickly, leaving one free to explore other aspects of the alignment, but something (perhaps the generation of input data for the tree calculation) takes a very long time and blocks the gui.