Create the input files for Adam Mazur's ShereKhan program.
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.
dir: The directory name to place ShereKhan cluster folders into.
This creates the files required for the ShereKhan server located at http://sherekhan.bionmr.org/. One file per spin cluster per field strength will be created. These will be placed in the directory `clusterx' and named `sherekhan_frqy.in', where x is the cluster index starting from 1 and y is the magnetic field strength index starting from 1.