In Momocs, Ldk classes objects are
lists of configurations of landmarks, with optionnal components,
on which generic methods such as plotting methods (e.g. stack)
and specific methods (e.g. fgProcrustes).
Ldk objects are primarily Coo objects. In a sense, morphometrics methods
on Ldk objects preserves (x, y) coordinates and LdkCoe are also Ldk objects.