
For 'mosaic' zero points, I define the mosaics by grouping the images
by observation time.  I do this by generating a list of the image
start times.  I sort this list and then find the unique entries.  Each
time defines a new mosaic entry.  I then assign images to the mosaics
by matching their start times.  

The above currently happens on the image subset.  I need to identify
the images which are NOT in the subset but for which a mosaic has been
constructed -- these are partially accepted images.  I then need to be
able to do something with those images and mosaics: either reject all
partial mosaics or accept all images from partial mosaics.  The
advantage of the latter is that the choice of accepted images depends
on the catalog / regions loaded -- I need to accept images only if all
overlapping region files are loaded, right?

So, a few things are needed: 

1) identify the images which are NOT selected by have matching start
times.
2) exclude them from the subset somehow.

Some questions:

 * the image subset is currently a *copy* of the Image array, not a
   view.  does it help to change this? (beyond the memory savings?)

 * I'm still a bit unclear on what 'load_images' vs 'reload_images'
   does.  

