Renamed AstroDataModel to ProcessRealsModel.
Added next_item method to ProcessRealsModel.
Determine total image count by number of source readings, not by
multiplying the number of sources by number of observations of 
current source, so that we don't rely on all sources have same number 
of observations.
Granularity of processed records increased from source level to reading
level.  These records are now stored as a dictionary.  Controller
updated to go to the next item instead of next source when accepting
or rejecting.