sdss/marvin: Marvin Beta 2.1.2

This patch release Fixes a major bug in the calculation of the composite spaxels in the BPT diagrams a bug for incorrect exception handling for some Python 3 installations of Marvin 2.1 Changes The NSA web parameters are changed from sersic parameters to elpetro parameters The NSA Display tab has been renamed to Galaxy Properties A link to the Nasa Sloan Atlas catalog page is attached on the web The BPT keyword snr has changed to snr_min. snr can still be used Documentation has moved to readthedocs Adds API and Web parameter validation using webargs and marshmallo