fixed photometric vs. spectroscopic cross-ID issue in astroquery/sdss

Previous version of astroquery.sdss.core.crossID required an object to have spectroscopy available in SDSS in order to count as a match. However, there are tons of objects with photometry available but no spectra. I've added the keyword argument 'spectro' in order to account for this. If True, SDSS spectra are required to count as a match. If False, objects with photometry but no spectra count as a match. False by default (more inclusive).