> However, I suspect we care more about fidelity of shearing accuracy than flux conservation, so I think we probably want to go 
> back to having Lanczos5 as the default and just point out the issues with the flux.

I agree.  It should be possible to clearly indicate that someone whose application requires perfect flux conservation should simply use a non-default interpolant.

However, I found that when I made this change (locally, haven't pushed it) then it breaks one of the unit test:
```

AIL: Test of operations on complicated InterpolatedImage: shear, magnification, rotation, shifting.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/EPD64.framework/Versions/7.2/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/rmandelb/great3/GalSim/tests/test_interpolatedimage.py", line 405, in test_operations
    err_msg = 'Size of magnified InterpolatedImage from HST disagrees with expectations')
  File "/Library/Frameworks/EPD64.framework/Versions/7.2/lib/python2.7/site-packages/numpy/testing/utils.py", line 468, in assert_almost_equal
    raise AssertionError(msg)
AssertionError: 
Arrays are not almost equal to 3 decimals
Size of magnified InterpolatedImage from HST disagrees with expectations
 ACTUAL: 13.502507206033709
 DESIRED: 13.50637149810791
```
This is a 0.03% error.  We have to decide if that's tolerable (in which case I'll change the unit test and push the updated code) or not.

> Also, I think Gary just derived the flux correction from a Taylor expansion type of calculation. 
> We might be able to figure out what the next term is to improve the flux conservation properties of Lanczos...

From his document (devel/modules/finterp.pdf) it seems like this should be easy.  I'm going to open an issue as a reminder.