Implemented back end for estimating r2eff and errors by exponential curve fitting with scipy.optimize.leastsq.
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 2014-08-24
Parent: [r25233]
Child: [r25235]
