Improved the checking in the relaxation curve-fitting analysis.
The new specific_analyses.relax_fit.checks.check_model_setup() function is now called prior to
minimisation and in the get_param_names() API method to prevent Python errors from occurring due to
missing data structures. In addition, the pipe_control.mol_res_spin module function
exists_mol_res_spin_data() has been replaced with check_mol_res_spin_data().
