The relaxation dispersion parameter errors from Monte Carlo simulations are now stored.
Previously MC simulations could run, but the errors were not being calculated and stored. The
sim_return_param() method was empty. This method is now complete.
In addition the set_error() method has been created for setting the parameter errors. And the
_exp_curve_key_from_index() and _param_index_to_param_info() auxiliary methods added to facilitate
data access.
