Modified user function 'relax_disp.r2eff_err_estimate', to be able switch between the Jacobians.
task #7822(https://gna.org/task/index.php?7822): Implement user function to estimate R2eff and associated errors for exponential curve fitting.
Authored by: tlinnet 2014-08-28
Parent: [r25394]
Child: [r25396]
