Swithced in estimate_r2eff_err() to use the chi2 Jacobian from C code, and Jacobian from python code.
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 2014-08-29
Parent: [r25428]
Child: [r25430]
