Implemented the first try to compute the Variance of R2eff and i0, by the co-variance.
This uses the Jacobian matrix. The errors calculated, are though way to small compared 2000 Monte-Carlo simulations.
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 2014-08-27
Parent: [r25328]
Child: [r25330]
