The CPMGFit input and output file names for relaxation dispersion are now MS Windows compatible.

This is needed to allow the files in the test suite to exist on Windows systems, as the '#:@'
symbols cause problems.  The same logic as used for the relax_disp.plot_disp_curves is used to
replace these characters with an underscore.
CPMGFit file name fixes for MS Windows.

The '#:@' characters have all been replaced by underscores.