New PNNL material compendium

[shimwell]

Super news that a new PNNL compendium has been released

paper https://www.osti.gov/biblio/1782721-compendium-material-composition-data-radiation-transport-modeling

pdf https://www.osti.gov/servlets/purl/1782721/

I think there might be a json format file somewhere

But this should probably be added to the NNM

[shimwell]

This could be parsed into the JSON format needed for NMM pnnl_15870-v2.mcnplibR2.txt