Fixes for all the N-state model optimisation unit tests.
Removed a debugging print out.
Fix for the func_2domain() method for the N-state model tensor reduction code.
Bug fix: The tensor reduction was specified back to front!
Added a check for the chi-squared value for the 5-state model system test.
Fix for the return_units() method.
The spin and spin_id args are now accepted.
Bug fix for the return_units() method, the spin_id keyword arg is now accepted.
Bug fix: The return_units() method now accepts the spin_id keyword arg.
Bug fix: The return_conversion_factor() method now accepts the spin arg.
Fix for the write_multi_header() function.
The spin_id variable now exists.