We want, from a 3d matrix, to extract a 2d (x,y) array whose each element
has been extract from a level specified by the 2d level array (typically,
we want to obtain the salinity along an isopycn we have repered by its level).
level2index is a function who give, in function of level, a 2d indexes array
which will allow to extract the 2d array from the 3d array...