Generate ELF data from primary sources

[jhidding]

The current ELF data is partly copied from tables in a paper book. This data should be available online somewhere. In interest of scientific transparency and provenance, we should obtain optical data from these public sources. We have such a database for X-ray data (Henke et al), and optical data is available in some places (http://refractiveindex.info/), but infra-red and far UV (or X-ray below 30eV) is very hard to find.