A mosaicker is an object which creates
a new image from a set of input images.
This mosaicker takes the input images and adds all the
overlaps together to create the output. It expects
to be able to get an ExposureFinder object which for
each input image returns the exposure for a given pixel.
After the contributions from all of the constituent
images have been found, the summed image is divided by the
exposure per pixel.
Substantially modified May 5, 2010 in response to issues
brought up by G. Belanger. Rather than have special code
to handle the overlap of each image with the output we
use the standard ImageFinder. This works in global geometries
where the prior use of clipping failed. The ImageFinder handles
the edge and radius clipping functionality that we had had to
duplicate here as well.