relax SVN archive

Instead of being part of the optimisation of the dispersion model, it will itself be the model named
'R2eff' (converted from the 'exp_fit' model). This model will either use the calc user function to
determine R2eff when the fixed relaxation period experiment is selected, or fit exponential curves
using the relax_fit C module for the variable relaxation period experiments. The R2eff values will
then be copied over for each dispersion model in the auto-analysis using the value.copy user
function.