Changes in XMBF 2.00:
--------------------

- added "parse_model" for fully user-defined models (see the XMBF manual for details)

- added the option "num_diff_first_order" in the "fit_settings" node. This
allows the numerical calculation of the first derivatives of the model functions
with respect to the parameters (intended for use with parse_model)

- added the options "second_deriv_covariance" and "second_deriv_minimization" in the "fit_settings"
node. These 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 manuals of XMBF and QMBF 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)