Gaussian filter

Added a wrapper around ndimage's Gaussian filter
    
This version of the Gaussian filter
    
 * uses 'nearest' as the default boundary mode. This can be discussed,
   but I had the impression that for images this is the most relevant
   mode ('extending' boundaries)
    
 * has a `multichannel` keyword, so that each color channel can be
   filtered separately. For now no attempt is made at guessing whether
   the image has color channels or not.