Make the scripts be less dependent on the 'gsl' and 'xfig' module

1) Fixed a missing variable declaration in the function
'continuum_correction' that prevented the scripts from
loading when the 'gsl' module was not available.
2) Replaced the function 'readascii' from the 'xfig' module
by a simple for-loop in the functions 'query_photometry'
and 'photometric_magnitudes'