Define format for line lists

The line lists added in #4 are taken verbatim from their sources and are thus somewhat heterogeneous. We should define a consistent format that is easily parseable and contains the necessary metadata like wavelength units and whether they're air or vacuum.

Start by reviewing the line lists added in #4 and comparing their source formats. Define a consistent, parseable format that preserves necessary metadata, including wavelength units and whether wavelengths are air or vacuum; done means the line lists follow that format.

Written by the indexing model from the issue text.