from a pipe and spins with optimised values of R2eff and i0.
The co-variance matrix can be calculated from the optimised parameters, and the Jacobian.
Big care should be taken not to directly trust these results, since the errors are quite different compared to the Monte-Carlo simulations.
This implementation, will reach the exact same error estimation as scipy.optimize.leastsq.
But with much better control over the data, and insight into the calculations.
task #7822(https://gna.org/task/index.php?7822): Implement user function to estimate R2eff and associated errors for exponential curve fitting.
