Intermediate step in estimate R2eff module.
It seems that minfx is minimising in a quadratic space because of the power of chi2, while the general input to scipy.optimize does not do this.
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 2014-08-25
Parent: [r25255]
Child: [r25257]
