Reviewer comments on exercises.

Dave,
So far, you've done very well!  I have a few comments that I've interspersed in your code.
-Sean

Incompatibilities between our systems:
   * You may wish to leave your local directories intact.
   * My local ds9 required using /Applications/ds9/ds9
   * I couldn't get showsrc working.
   * I didn't have your files from MAST