#333 - new GSObject, InterpolatedImage

This pull request is for work to create a new GSObject, the InterpolatedImage, which has an SBInterpolatedImage as its SBProfile attribute.  It is initialized from an Image, with keywords to tell it which normalization convention was used for the Image (i.e., is it an image of flux, or surface brightness).

A few points to note:

* Mike made the InterpolatedImage usable from config, which requires specification of a filename containing the Image to be read in and turned into an InterpolatedImage.

* He also changed the default parameters of the Lanczos interpolant to be ```conserve_flux = True``` and ```tol=1.e-4``` (from ```False``` and ```1.e-3```) because every time we use it in practice, we were setting those parameters anyway.

I tried to make a decent set of unit tests, but as always, comments on those are welcome.