relax SVN archive

Modified all the model-free optimisation system tests to check if the spins are properly selected.

Changed 'unselected' to 'deselected'.

Bug fix in the test_opt_constr_sd_back_S2_0_970_te_2048_Rex_0_149() system test.

'spin.chi2' was being accessed as 'spin.chi'.

Modified the model-free minimise() method to use the new return_spin_from_index() function.

Created the new function generic_fns.selection.return_spin_from_index().

Missing import of return_spin().

Bug fix in the minimise() method for the call to diff_data_exists().

Converted the model-free minimise() method to the new relax design.

Created a new function count_selected_spins() for counting selected spins.

Another bug fix for determine_param_set_type().

The structure cdp.diff_tensor should have been accessed, not cdp.diff.