relax SVN archive

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.

Bug fix for determine_param_set_type(), the diff_data_exists() function is properly called.

Updated the model-free determine_param_set_type() method to the new relax design.

assemble_param_vector() now calls determine_param_set_type() to get the model type.