relax SVN archive

Checking has been added for the model-free return_units() method for the presence of frq_labels.

The spin is passed into the return_data_desc() method.

The object name and description list is build for each spin separately now.

Fixed a call to return_units().

Updated the return_units() method to accept the spin arg.

Added the spin arg to the return_data_desc() method.

Bug fix for the xml_create_element() method for when the data_names() function is None.

The units in the return_data_desc() now come from the return_unit() method.

Shifted the spin specific object name and descriptions into the specific code.

Added a few more specific functions to get_specific_fn().

The spin specific relaxation data structures are now properly initialised.