Customized spectral lines

Would be good to let the user have his own custom files of emission and absorption files, instead of the default one.

(Vanina Ruhlman's request)

Start by locating where the default emission and absorption files are loaded in the Python visualization tool. The issue names no files, tests, or entry points, so map the current loading path and ask maintainers to confirm the expected user-supplied file interface. Done means custom emission and absorption data can replace the defaults without breaking existing behavior.

Written by the indexing model from the issue text.