Create the input files for Michael Bieri's NESSY program.
dir: The directory to place the file and to use as the NESSY project directory. If not given, this defaults to the current directory.
force: A flag which if set to True will cause the files to be overwritten if they already exist.
spin_id: The spin identification string.
This will create a single NESSY save file called `save.NESSY'. This will contain all of the dispersion data currently loaded in the relax data store. If the directory name is not supplied, this will default to the current directory.