Redesigned the peak intensity loading GUI wizard for handling multiple experiment types.

This follows from http://thread.gmane.org/gmane.science.nmr.relax.devel/4530, the thread about
supporting multiple data types such as SQ+MQ data simultaneously.

The logic for the page ordering needed to be changed to be more dynamic.  The CPMG and R1rho pages
need to be shown only if the corresponding experiment type exists in the current data pipe.  Hence
the has_cpmg_exp_type() and has_r1rho_exp_type() dispersion functions are now used by the new
methods wizard_page_after_relax_time() and wizard_page_after_cpmg_frq().  A number of now useless
flags have also been removed.
Added some sanity checks to the dispersion target function class.

R1rho models cannot be used with CPMG-type experiments, and CPMG models cannot be used with
R1rho-type experiments.
Fix for a bug introduced at r20855.
Fixes for all of the GUI dispersion tests for the changes to cdp.exp_type.

This follows from http://thread.gmane.org/gmane.science.nmr.relax.devel/4530, the thread about
supporting multiple data types such as SQ+MQ data simultaneously.
Large speed up of the Relax_disp.test_tp02_data_to_tp02 GUI test.

By minimising the number of times the peak intensity wizard pages are regenerated, the test time
decreased on one system from ~32s to ~12s.