Handling graveyard volumes

[pshriwise]

At the moment, I've opted to handle graveyard volumes inside the MOABMeshManager class, so it doesn't exist as a concept outside of that class. Any graveyard volumes discovered in the MOABMeshManager class will automatically have its boundary conditions set to vacuum and material to void. The underlying DAGMC model's metadata is unaffected by this.

Overall, I like this as the terminology applies specifically to MCNP models. However, there may be cases where we don't want to do this and it might be good to make it optional.