Rename 'flux' column in tables generated by ``StarFinderBase`` objects

[mirca]

I think it makes sense to rename the flux column in tables generated by StarFinderBase objects to something like flux_peak_density. I'm saying this because the name flux induces users to use those numbers as the initial guesses for the flux parameter in the PSF photometry code.

P.S.: I myself used flux from those tables as initial guesses for PSF photometry which end up generating bad results because those values were too far from the "true" flux value, thanks @eteq saw that and we end up using aperture photometry to generate quite good guesses.

[larrybradley]

@mirca Yes, I agree with that this will limit confusion. However, this change should apply only to DAOStarFinder. The output flux from IRAFStarFinder actually is a background-subtracted flux (I think the original IRAFStarFinder docs got overwritten when we moved them to the StarFinderBase class). In other words, the find_stars documentation needs be different for the two finders. I can work on this since there are other things I want to clean up in the star finders.