Orbfit raises custom OrbfitError instead of generic Exception.
Print out new orbit fit & orbit fit quality after accepting an image.
Merge pull request #230 from drusk/orbfit_info_on_accept

Closes #225.
Added 'succcess' tracking to update_header so retry only happens on previously failed header updates.  Required to start dealing with issue #231
Modified zscale call to try to get more images displaying; still doesn't grab May data. Also add origin file to output .mpc files to help in data linking.
Merge remote-tracking branch 'origin/master'
 Closes #227 : made orbfit and validate aware of 'null_observation' setting.  Nulls are still displayed, but with a 'red' circle to indicate they are nulls, and nulls are no longer used in the 'fit'.
Merge remote-tracking branch 'origin/master'
corrected setting of null_observation on Reject
Now sends x/y centroid back from daophot, also prompts if daophot centroid differs by more than 1.5 pixels from marked position, user can choose which one to use.  Closes Issue #216
closes Issues #222 and #228 by removing of zscale modification (problem was the data, not the viewer).  Also fixed up requery so that works inside the application well, mpc filename is now bumped as versions change.  Added optimization so existance checking is quicker and mopheader is cached.
closed loop on finishing a 'track', now you move to the next one.  Also cached some info so no-need to require vospace so much.
corrected issue where ssos query looped forever.
orbit fit now provides heliocentric distance, length of arc in days
wanted a rough hack at motion rate & accuracy for April next year. These are hardwired for analysis of E block only; generalise later...