check.selection.strength checks, if the process is an OU, if the
selection strength is not too low, in which case the process is replaced
with a BM.
check.selection.strength(process, selection.strength = NA, eps = 10^(-6),
...): "BM" or "OU"
the selection strength parameter (if OU)
the tolerence for the selection strength
character : "BM" or "OU"
This function return a process in a form of a character. If the entry process is "BM", then nothing is done. If it is "OU", then the verification of the selection strength is done.