Extracted Displayable superclass from DisplayableImageSinglet and 
DisplayableImageTriplet.
Created _AxesItem class which handles operations related to a matplotlib
axes.  Removes a lot of the duplication between the singlet and triplet
displayable items.
WIP: reworking image triplets to use individual axes for each subimage.
Doing this by extracting an ImageSinglet class from the 
DisplayableImageSinglet and composing the DisplayableImageTriplets
of 9 ImageSinglets.  This also allows a greater degree of code re-use.

Markers are temporarily disabled as their calculation is moved out of the
controller into the fitsviewers.  Displayable items are also no longer 
created at the model level, they are created by the fits viewer from a 
cutout or a cutout grid.
Added back markers for singlets, implemented in the fitsviewer.
Added method to apply a function to each element of a CutoutGrid.
Added markers for triplet view.  They are currently centered on the source
in each image instead of in the source in the main image.
Triplet markers are now placed on the location of the source in the 
'main' image of the frame.
Removing unused code.
Removing unused code.
Added ability to put spacing between triplet images.
Removed redraw flag from fitsviewers' display method.  That should be
taken care of internally.
Updated test case.
Updated test case.
Updated test case.
get_rect width and height calculation should now work for general grids.
Skipping orbfit test until merged with track branch.
Added black border around images instead of spacing because it seems 
to be more clear.
Updated marker test for image singlet.
Fixed reset colormap for singlet viewer.
Added fits_header and astrom_header properties to SourceCutout.