Allow implicit complement material assignment on empty group/block

[pshriwise]

As it stands, an implicit complement material can only be assigned via a graveyard volume. It seems more natural to me to assign this material via an empty group/material with the name "mat:<material>_comp" without requiring a graveyard volume to exist. This would be particularly useful in OpenMC models where a user would like to specify a "background" material for a DAGMC universe that fills a cell. In this use case it would only add additional confusion to include a graveyard volume solely to assign the implicit complement material.

This limitation is noted on the website here,

Since the implicit complement doesn’t exist before running DAGMC, and DAGMC can only recognize groups that contain an entity, the material for the implicit complement must be specified as if it were being specified for the graveyard volume

but I think this is something we can find a way to work around to streamline this capability.