Switch between fixed or fitted R1 values for optimisation.
fit: The flag specifying if R1 values should be optimised or if loaded R1 values should be fixed during optimisation.
This user function allows the optimisation of R1 values to be turned on an off for the relaxation dispersion dispersion models. If turned off, the current values of R1 will be fixed. Otherwise the R1 values will be added to the model parameter set. For models which do not support the R1 parameter for off-resonance effects, this setting will have no effect. Only the models [`No Rex', `DPL94', `TP02', `TAP03', `MP05', `NS R1rho 2-site'] support R1 optimisation.