Revise photometric fitting

The photometric fitting procedure is revised in a few aspects:
1) Most importantly, the function 'photometric_magnitudes' now
   computes magnitudes in the AB (absolute) magnitude system for
   all photometric systems. Conversion to other systems such as
   Vegamag can be done via zero points, which are introduced
   with this commit, e.g., as qualifiers for the function
   'photometric_magnitudes'. All functions affected by this
   change have been revised accordingly. Uncertainties in the
   zero points now provide a mechanism to account for systematic
   uncertainties in the photometric fitting procedure because
   these zero point uncertainties are added in quadrature to
   the given uncertainties.
2) The filter response curves of the following systems have been
   updated: Johnson-Cousins, Stroemgren, Geneva, SDSS, Hipparcos,
   and Tycho.
3) The qualifier "remove_outliers" of the 'photometric_fitting'
   function now works differently: Instead of being set or not
   set, it now has to be a number that specifies the sigma thres-
   hold above which outliers are removed.
4) The function 'filter_passbands' no longer accepts three arguments
   because this feature was not used at all.
5) The photometry template has been updated. In particular, first
   guesses for the parameters 'E_BmV' and 'logtheta' are better now.