Fix for the display of the spin-lock nu1 values in the dispersion GUI tab.

This was reported by Troels at http://thread.gmane.org/gmane.science.nmr.relax.devel/4708.

The GUI spectrum element at gui.components.spectrum was at fault, the add_disp_point() method was buggy.
Fix for the right click pop up menu entry "Set the spin-lock field" in the dispersion GUI tab.

This is for the spectra list relax_disp.spin_lock_field user function call.  The reference spectra
are now detected and the field value set to None.  This fix has been propagated to the
relax_disp.cpmg_frq user function menu entry as well.