Mustang sed_skinstress
A redesign of sed_skinstress subroutine https://gitlab.inria.fr/croco-ocean/croco/-/blob/master/MUSTANG/sed_MUSTANG_CROCO.F90#L433 will permit to :
- homogenize variable names with english names (change tenfon*, frofon*...) (see also #32)
- highlight more clearly the user choice in the code from each cppkey : BBL, key_MUSTANG_tenfonUbar, key_tenfon_upwind, WAVE_OFFLINE
- update documentation of mustang on this subject