Add PNNL 15870 to openmc.model

The document in the title is here.

It contains a huge list of standard material compositions for radiation transport calculations. This would fit nicely into openmc.model.

Note that PyNE includes this data in their material library. It might already be possible to get a corresponding OpenMC material since their Material class has a openmc() method that produces XML (which we could parse with openmc.Material.from_xml_element?).

Obviously being able to get this from openmc.model would be way simpler for a user.

Ah, good idea. Maybe we could just get the XML from PyNE, and include it in openmc.model?