current experimental changes
see changelog

- added fit_powerlaw_individual, which uses the individual data points in
  the continuum fitting intervals, instead of the median values
- to compare both fittings, PyS_SDSS-fit_compare is added
  receives input file with list of FITS files and plots spectrum
  of each, with both fitting procedures used
- added --deviation_factor argument option on startup in SDSSclasses
  and on call of fit_powerlaw_individual, which is the factor*std
  which the flux can deviate from the mean in the fitting intervals
  in order to be taken into account
- implemented experimental (currently commented out) version in build_compspec
  which is a lot faster, but currently still produces wrong results
- probably other smaller changes