Function to calculate T_bar.
tbar(partition, structure_matrix)two-dimensinal matrix of mode numeric with
rows as types, columns as subcommunities, and elements containing relative
abundances of types in subcommunities. In the case of phylogenetic
metacommunities, these are the relative abundances of terminal taxa.
output$structure of phy_struct(); each
row denotes historic species, columns denote terminal taxa, and elements
contain branch lengths.