Changes in QMBCFN 1.40:
----------------------

- 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 to avoid crashes)

- additional warning messages if incorrect settings are entered in the "Fit Settings" tab

- fixed a bug in the GUI concerning the hiding/showing of the lineedit fields for the derivatives when numerical differentiation is enabled