relax SVN archive

Added the ability to store and return conversion factors in the Param_list specific API object.

This includes accepting functions so that factors requiring user loaded data can be supported.

The relaxation curve-fitting specific analysis now uses the new SPIN_PARAMS list API object.

Converted the NOE specific analysis to use the new SPIN_PARAM list API structure.

Added the 'select' and 'fixed' objects to the base SPIN_PARAM list.

The Param_list.get_desc() method now returns None rather than a RelaxError if the param is missing.

Created the specific function API common method _return_data_desc_spin().

Created the Param_list.get_desc() method for returning the parameter description.

Removed the spin arg from the return_data_desc() specific function API method.

Removed the spin and spin_id args from the specific API return_units() method.

This was needed for the model-free Rex parameter where the first field strength was part of the
units. This has been replaced by the first global frequency value.

Removed the spin_id set to None check in the _set_param_values_global() specific API common method.

This check was not functioning correctly.