Changes in MBF 5.20:
-------------------

MBF was updated to reflect the changes in QMBF 5.20:

- added features that allow the numerical calculation of the second derivatives of the model functions
with respect to the parameters, as needed to get the complete second derivatives of chi^2. See the user
manual for more details.

- the Hessian of chi^2 is now checked for positive definiteness after the end of the fit
(needed because of the new features concerning second derivatives)

- improved the handling of numerical exceptions (more warning messages; GSL error handler is now turned off)