Allow updating the PNNL material list used by the openmc app

[moellep]

The openmc app uses an hdf5 file with PNNL data created (pkcli.openmc create_standard_materials_file()) from the json compendium.

The compendium source is here:
https://github.com/pyne/materials-compendium/blob/develop/src/materials_compendium/MaterialsCompendium.json

And the extracted info used by the openmc app is:

Name
Density
Elements
  Element
  AtomFraction_whole
  Isotopes
    Isotope
    AtomFraction_whole