Added front end user function relax_disp.r2eff_estimate() to estimate R2eff and errors by exponential curve fitting in 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: [r25231]
Child: [r25233]