Function to calculate T_bar.
tbar(partition, structure_matrix)two-dimensional matrix of mode numeric
with rows as types (species), columns as subcommunities, and each
element containing the relative abundance of types in each subcommunity
relative to the metacommunity as a whole. In the phylogenetic case, this
corresponds to the proportional abundance of historical species, which is
calculated from the proportional abundance of terminal taxa
output$structure of phy_struct(); each
row denotes historic species, columns denote terminal taxa, and elements
contain branch lengths.