This only affects the model-free analysis. The pipe_control.diffusion_tensor module functions
default_value(), return_conversion_factor(), return_data_name() and return_units() have been
eliminated. These functions have been merged together with the diffusion parameter self._add()
calls in the model-free specific parameter list into the new parameter list base class
_add_diffusion_params() method.
To allow the model-free analysis code to differentiate between diffusion and model-free parameters,
the new scope() parameter list base class method has been created. Diffusion parameters return
'global' whereas model-free parameters return 'spin'.
The model-free specific API methods default_value() and is_spin_param() have been deleted as these
are now provided fully by the parameter list object. The is_spin_param() method has been newly
implemented in the parameter list object to check the parameter scope.
