[WIP] longslit wavecal

DO NOT MERGE
This PR integrates the longslit wavelength calibration with the geminiutil framework.

There are still several tasks to be done:

- [ ] finish the writing to HDF5 table
- [ ] finish the storing of the wavelength solution in the database
- [ ] add the ability to use a dataslice on the calibration (and remove method `prepare_longslit_arc`)
- [ ] update documentation