Feature: Switch default library from JEFF 3.1.2 to ENDF/B VII.1

[yardasol]

Background and motivation

JEFF 3.1.2 is an old library and has many bugs in it. It also does not play nice with OpenMC.

Description of idea

We should change our default library from JEFF 3.1.2 to ENDF/B VII.1, which seems to be widely used. We'll specifically want the LANL version of endf, as this is readily available in ACE format. OpenMC provides a downloadable version of this library as well. We can download it here

Implementation details

I have already created a script to download and set up the ENDF library in this PR.

Potential snags

The biggest issue with doing this is that it effects our integration tests. We don't have to switch them right now, but later down the line it may be beneficial to do so.