$slidings in Ldk must be a 'valid' matrix: containing
ids of coordinates, none of them being lower than 1 and higher the number of coordinates
in $coo.
slidings matrix contains 3 columns (before, slide, after).
It is inspired by geomorph and should be compatible with it.
This matrix can be passed directly if the slidings argument is a matrix. Of course,
it is strictly equivalent to Ldk$slidings <- slidings.
slidings can also be passed as "partition(s)", when sliding landmarks
identified by their ids (which are a row number) are consecutive in the $coo.
A single partition can be passed either as a numeric (eg 4:12), if points
5 to 11 must be considered as sliding landmarks (4 and 12 being fixed); or as a list of numeric.
See examples below.