Added module gfs_mod / ecmwf_mod to allow compilation of different versions without editing source code

If the 'dates' file exists on simulation start, backup to 'dates.bak' and create new file 'dates' instead of appending to it

Added a modification so thah number of particles in simulation reported to standard output (MPI version) is exact instead of estimated

Move OH_FIELDS directory to flexpart../flexin/OH_FIELDS; this is the default location, it should be possible to offer an alternative path in the COMMAND namefile