Started to redesign the relaxation dispersion target function class.
The input data is now expected to be R2eff/R1rho data and all mentions of exponential curves have
been eliminated. The func_exp_fit() target function has been deleted as it is not used - as now
the _minimise_r2eff() method in the dispersion specific analysis class is used instead. And the
func_fast_2site() target function has been renamed to func_LM63().
