relax SVN archive

Fix for the model-free model_loop() generator function.

Removed some unused imports of return_spin_from_index().

Deleted the set_selected_sim() methods, as the base class method is sufficient.

Removed some dud arguments to the model_loop() generator.

Wrote the model-free model_loop() generator method.

Docstring and comment fixes for select_all_sims().

Shifted set_selected_sim() to the base class.

Updated the set_selected_sim() method.

Modified the select_all_sims() function to use the model_loop().

Propagation of the spin_id variable.