#442 Improve Lanczos DC flux conservation

This PR is mostly for my work in improving the ability of our Lanczos interpolant to preserve a constant flux for a DC input image using the `conserve_flux` parameter.

It turns out that the correction sceme we are using gets progressively less effective as you add more terms, so even though I went from 1 correction term to 5, I only increased the accuracy from about 4e-4 to 7e-6.  Still, since we often target 1.e-5 for many or our accuracy items, this seems like a worthwhile improvement.

Another change on the branch is that I made RealGalaxy and OpticalPSF use the same default interpolant as we set in InterpolatedImage.  We had been having RealGalaxy use Lanczos-5 by default, and the other two used Quintic.  I think we want these to be regularized.  

I picked Quintic for the common default interpolant because there was a test in test_interpolatedimage.py that failed when I used Lanczos-5.  Specifically, the magnification test.  Melanie also found that Lanczos doesn't do a great job on her magnification tests in branch #389.  However, there are other tests where she found Lanczos to be better than Quintic, so we might reconsider this at some point.  But with Quintic, all the unit tests pass.

Finally, I moved the unit tests that were in test_spline.py into test_interpolatedimage.py and recast them as reference tests rather than tests of the spline routine, since most of them now don't use table look-ups.  We can think about whether we actually want to make some unit tests of the spline function in Table now that it's wrapped into python (as LookupTable), but I think this doesn't need to be very high priority.