
I am making some fixes to the code to select an image from the full
table (specifically for mextract).

here is how things work currently: 

* mextract detects that a query needs data from an image field
* mextract calls SetImageSelection()
  * SetImageSelection calls:
    * LoadImagesDVO(), which reads the image table into its static cache
    * BuildChipMatch to generate the index for chip->mosaic 
    * image_subset which generates a local (to ImageSelection.c) static subset
    * sort_image_subset sorts the subset selection to be ordered by time
* mextract calls MatchImageDVO to find a specific image
  * match_image_subset is ok because subset is sorted by time

* for the image metadata, I need to do the following:

* load the image subset table
