Simplified the code of the Relax_disp.test_hansen_trunc_data GUI test.
The GUI now warns if the user choses inappropriate models.

When clicking on 'Execute', an error message appears if R1rho models are selected for CPMG data and
vice versa.  This is simply for more intuitive user feedback.
Fix for the relax_disp.exp_type pop up menu entry in the spectrum list GUI element.

This was calling relax_fit.exp_type rather than relax_disp.exp_type.
Fix for the relax_disp.cpmg_frq pop up menu entry in the spectrum list GUI element.

The method associated with the menu entry action_relax_disp_cpmg_frq() was buggy.
Fix for the relax_disp.spin_lock_field pop up menu entry in the spectrum list GUI element.

Another action method bug - the same as in the last commit.