Tool is needed to perform adaptive smoothing of images

[DaveStrickland]

A python-based class and command line tool is needed to perform adaptive smoothing of processed images, in particular resampled composites of multiple images. I'm using Jeremy Sanders' contbin command line package (C++, cfitiso) locally, but we need a python-native solution (see #3).

• The size of the smoothing kernel should depend on the local signal-to-noise.
• It should be able to include noise terms from read noise, bias/dark subtraction, and any additional sky subtraction.
• It should be able to handle exposure maps.
• High S/N regions should not be smoothed into lower S/N regions.
• No external non-python dependencies.
• Calculation of the smoothing kernel size as a function of position should be separate from the actual smoothing.