added easy linking - need for rebase
removed additional kwargs for link_science_frames
first changes towards a workdir
changing all file storing columns to use AbstractFileBase
fixing arcs to be not AbstractFileClass to be able to point outside the workdir to the calibration directory
some fixes
Merge pull request #51 from wkerzendorf/gemini/add_main_workdir

Gemini/add main workdir
science_frame -> science_set
added extraction into slices again
added warning if science set science frame is not prepared
added function to get slice from mask arc
added AbstractCalibrationFileTable that has its workdir automatically set to the data directory
fixed the relative paths in the gmos project - also moved the datafiles from gmos/data to data/gmos
got rid of prepare_science_sets as it does not seem to be needed.
updated the arc to the full line list
updated the path for some calib files
some final fixes - seems good to ship.
fixed minor mistake
quick change for AbstractCalibration...
Correct calculation of tracepos in extract_spectrum