Changed name to cluster, made fitting iterative

Changed the image class to cluster class. No changes to the class
itself, only the name.

Also made the red sequence fitting process iterative. Before it fit on
just the galaxies within 5 sigma of the initial redshift. Now it does
that, then selects galaxies within 4 sigma of that redshift, fits a
redshift to those, then fits again on galaxies within 3 sigma of the
newest redshift.