Added to back-end of R2eff estimate module, to be able to switch between the function Jacobian or the chi2 Jacobian.
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: [r25393]
Child: [r25395]
