changed the way 'null_observation' logic is handle.
changed the init method to include full orbit fit initialization. Also improved the way that residuals and arc-length are computed.
re-worked to first use 'vos' and then 'requests'.  Requests will only work if we are not dealing with 'links'
added some error robustness to mpcread, not completely correct but close.
changed the 'B' limit on stars returned by usno query.  also removed some verbosity.
Some PEP clod cleanup.  Made the search for various location an image could be into a loop instead of series of 'ifs'.