add effective radius to the summary catalogs

From @djschlegel:

I suppose the obvious old-school quantities to report
would be the Kron radius, Petrosian radius, and effective radius.
All those are in principle independent of surface brightness dimming
or extinction.

The sextractor radius is something like a Kron radius, yes?

I agree with Marla that if folks still want these values, it would be better
for you to just compute them uniformly with some set of assumptions.
That’s better than have an N-body simulator try to figure it out, for example.

We could kibbitz on those choices.
— Measure straight off the SB profiles only?  Another choice would be
     to measure directly on the image pixels where the s/n is high, and
     the SB models further out
— Subtract foreground stars and galaxies.  Of course, measuring
     directly on the SB profiles does this.
— Do you try to determine an error on those?  Probably not.
     If so, that’s probably dominated by the sky level estimate...

moustakas / SGA

add effective radius to the summary catalogs #41