Fixes and improvements to the model-free model_statistics() method.
Fixed the call to return_units().
Converted the model-free return_units() method to the new design.
Docstring fix in return_conversion_factor().
Fixed the call to return_conversion_factor().
Completed the map_bounds() docstring.
Converted the model-free return_conversion_factor() method to the new design.
The proton type is now set in all the model-free OpenDX system tests.
Fixed the call to generic_fns.value.set().
Fixed the references to self.file_prefix.