Calculates a a distance matrix
from a tree in Newick format, which must have
branch lengths specified. The distance between two leaf nodes in the tree is
calculated as the sum over branch lengths of the shortest path between leaves.
The distance matrix is written to the filename given by
the -distmxout option. See distance matrix for format
usearch -tree2distmx tree,phy -distmxout mx.txt -format
tabbed_pairs -distmxo fractid