Adding a sky spectrum

It may be useful to add a sky spectrum so that one can check whether or not emission line features of objects are at the wavelengths of strong sky emission lines. This can give some hints about whether observed emission line features are due to residuals of sky lines.

The issue names no files, tests, or entry points. Start by locating the existing spectrum data-loading and visualization paths, then determine how a sky spectrum should be represented and compared; done means strong sky-emission wavelengths can be inspected alongside object emission features.

Written by the indexing model from the issue text.