better handling of off-target images
 - if an image does not contain the target RA,Dec. position then we put it
 into epoch -1 and set the (new) exp.ontarget flag to False.
 The "working exposure list" that is defined after the epoch sorting stage
 discards those off-target epoch -1 images, and so they are never available
 for subsequent processing.
added .e to the ignore list
Added debug option to exposures.checkonimage
bug fixes for epoch sorting and ref image construction
  * epoch definition step (and therefore the generation of the _epochs.txt file)
    now includes all epochs, even when using the --epochs flag to limit the epochs
    that actually get processed
  * refimage construction no longer fails while looking for the deepest visit
    in the ref epoch and filter when the refepoch is not included in the
    list of epochs for processing.