Echelle light / dark pairing

[planetarymike]

To run the BU echelle pipeline, we need to pair light and dark l1a files. For most of the mission this is easy, but early on there are some strange observations that are mislabeled.

We need:

• an is_dark routine to inspect the echelle l1a file contents and determine if the file is actually a light or a dark, regardless of the filename.
• code to pair light l1a's to the appropriate dark (mostly written, in mike's echelle_scratch notebook)
• comparison with Majd's list of manually paired light and dark files