Gmos/dayarc classes

@wkerzendorf - With this, dayarc calibration is relatively simple (and hopefully easy to improve further; as is, red arcs need some fiddly initialization parameters, which almost all have to do with line finding and matching -- hence my interest in doing it with the cross-correlation type method):
```
daycalfits = [fil for fil in proj.daycal
              if(fil.object.name == 'cuar' and
                 fil.fits.header['maskname'] == '0.5arcsec')]
prepdayarc = GMOSPrepare(bias_subslice=[slice(None), slice(1,11)],
                         data_subslice=[slice(1150,1250),slice(-1)])
calbluedayarc = GMOSDayArc(line_catalog=lincat['w'] * u.Angstrom)
calbluedayarc(prepdayarc(daycalfits[0]), doplot=True)
```