relax SVN archive

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 This has been replaced by the first global frequency value.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.

Started to migrate the frame order analysis to use the SPIN_PARAMS and GLOBAL_PARAMS specific API objects.

Created a few additional specific API common methods.

These include:
_return_data_name()
_return_data_name_global()
_return_units_global()

Created the structural API GLOBAL_PARAMS structure.

This is also based on the Param_list object and is initialised in the base class for all analyses.