Added method to coordinate converters to obtain a second converter 
which makes the transformation back from the original's output 
coordinate system to its input coordinate system.
Added method to DownloadedFitsImage for converting from pixel 
coordinates to the observed coordinates.
Made distinction between pixel source location and observed source 
location in the model.  Pixel source location is the coordinates of the 
source in the cutout image, while observed source location is in the 
coordinates of the original image (as used in the astrom file).

Calculating observed magnitude with daophot uses the pixel source 
location whereas calculating RA/DEC with xy2sky uses the observed source
location.