Several changes to handle standard FITS spectra files

1.  Added DC-FLAG handling
2.  Dealt with BZERO (maybe not quite right..)
3.  Added error file to read_fits.read_fits_spectrum1d
@keflavich has a good point - however, the extension is difficult, as sometimes the WCS header is in extension0 while the data is somewhere else. We definitley should not hardcode and extension, for a particular file.