Add general ENDF depletion chain script

[drewejohnson]

In light of some discussion in #19, I think it would be more useful to provide a general ENDF depletion chain generator. This script should

1. Accept ENDF distributions back to VII.0 and up to VIII.0
2. Fetch the correct decay and neutron induced fission files (hash checking strongly encouraged)
3. Locate (via download or local directory) neutron reaction files
4. Pass evaluations on to openmc.deplete.Chain.from_endf

Care should be taken with the VII.0 decay library, as it comes out as a single file (with negative uncertainties in cobalt 55).

In the future, this could replace the openmc-make-depletion-chain script that currently lives inside the main openmc repo