Enhancements

[PJ-Watson]

• Optional colourbar for beam plots.
• Include a quality flag for the redshift fit.
• Prevent looping around the list of objects (or raise a message when the end of the list is reached).
• Raise a warning/message the first time the 'next' or 'previous' buttons are pressed, to check that the correct read/write mode has been selected.
• Add option in config to adjust filename output.
• Option to limit selection to a set of IDs/coordinates?
• Separate R and C grism spectra (PASSAGE).
• Add save button for beam view.

Self-assigning this in order to keep track of progress.