Set up to run with DIS.

[zkbt]

This is almost there. We at least have the R300 (low-resolution, red arm) grating working. I still need to

• add a wavelength calibration file for the B400 (low-resolution, blue arm)
• change the extraction defaults a bit (wider sky-subtraction region by default)
• test out a bit more thoroughly on an actual transit dataset

[zkbt]

This is mostly accomplished by #17. I still want to tidy up the little extra features above, but it should be easier to merge with IMACS now.