Include calibrations

[Russell-Ryan]

We will need to introduce the calibration data. This is sure to include (but may not be limited to):

1. true trace/dispersion models. This will give the notional positions of the trace/dispersion, but importantly provide the mapping from "position along the trace" (r) to the wavelength (lam)
2. sensitivity curve. This will convert from detector units (DN/s/pix) to physical units (erg/s/cm2/A). At present it is unclear if this will include the dispersion (dlam/dr), so care should be taken to verify this.

Additional things to keep in mind, depending on the state of the data:

1. flat field.
2. sky background (ie. not the host galaxy)
3. astrometry.