The function returns a list object. If no mismatch between
tree and y is detected, the list only includes the matrix of
y ordered according to the order of tips on the tree ($y).
If some tips on the tree are missing from y, they are
removed from the phylogeny. Thus, the list also includes the pruned tree
($tree) and the vector of dropped tips
($removed.from.tree). Similarly, if some entries in y are
missing from the tree, the list also includes the vector of
mismatching entry names ($removed.from.y). In this latter case, the
first element of the list ($y) does not include the entries
$removed.from.y, so that it perfectly matches the phylogeny.