relax SVN archive

Parameters can be fixed to the original values during the frame order grid search.

The grid_search() user function argument incs can now be a list with None elements to fix parameters.

This will only be used in the frame order optimisation for now.

arg_check.is_int_or_int_list() can now allow for None list elements.

The model can be overwritten by frame_order.select_model().

Missing relax error import.

frame_order.ref_domain() now only updates the model info if the model is set.

Removed the model check from frame_order.ref_domain().

Fix for the test_compile_2nd_matrix_rotor_point1() unit test.

The compile_2nd_matrix_rotor() function was being incorrectly called.

Added 2 system tests for the rotor frame order model.

The rotor frame order model is now functional.