shifts.list_to_matrix takes the list description of the shifts
to give the matricial representation of the shifts : the b th element of
the lth line has the value of the shift on character l occuring on that branch b
shifts.list_to_matrix(phy, shifts, p = nrow(shifts$values))Input tree.
list description of the shifts : shifts$edges, shifts$values.
number of traits (optional, needed when shifts = NULL).
Matrix p x nEdges of length nbranch.