Made the user function, which estimates the R2eff errors, use the Jacobian derived from chi2 function.
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: [r25404]
Child: [r25406]
