Fix for the Monte Carlo simulations for the dispersion auto-analysis failing under certain conditions.

The wrong variable was being checked to see if more than two models were being optimised.
The dispersion minimisation() method now skipped deselected spin clusters.

This is defined as all spins of the cluster being deselected.
Implemented the new relax_disp.parameter_copy user function.

This is used to copy relaxation dispersion parameters from one data pipe to another, taking cluster
averaging into account.  It is used by the dispersion auto-analysis to handle clustering.
Added an element to the dispersion GUI analysis for specifying the directory of previous results.

This is used for the pre_run_dir argument for the dispersion auto-analysis.
Reactivated Monte Carlo simulations for the 'R2eff' model for exponential data curves.

This is within the optimise() method of the dispersion auto-analysis.