Manage isnan non standard fortran intrinsic function with wrapper that switch to ieee_is_nan when preprocessing key CPP_USE_IEEE_IS_NAN is set.
This is mandatory for nvdia compiler that is not accept the isnan function. Due to not well understood side effect with MPI initialization on irene, for now we offer the 2 possibility, and by default, CPP_USE_IEEE_IS_NAN is set only for nvidia compilers.
YM