updated calculation for slice positions
in the middle of updating the slice cross correlator. switching now to get science frames associated with their calibration data.
changed date-obs to mjd using astropy time
added the possibility of using the .science_query option to get the query object back
1/0 in there.  building the flat, arcs, ... finder.
fixed the instrument_setup from_fits_file classmethod. it was returning not the same instrument_setup even if it did exist already.
added mask_arc and flat_id
finalized the science frames
Merge pull request #28 from wkerzendorf/gmos/science_frame_table

Gmos/science frame table