Implemented the Jacobian of exponential function in Python Code.
This now also gets the same error as leastsq and C code.
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: [r25338]
Child: [r25340]
