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.

[pshriwise]

As we revisit this topic in svalinn/pydagmc#57 (specifically this comment, I wanted to add that I'm recalling the origin of this issue being that Cubit will not export empty groups. So the requirement that the graveyard volume be part of this group set (or exist at all) may not be a hard requirement. I'll try to verify this.

[pshriwise]

Using this pincell model from the OpenMC test suite I used pydagmc to remove the graveyard volume from the group labeled mat:water_comp, wrote it out, and confirmed that the implicit complement material group still existed after reloading the file.

Based on the change in the resulting eigenvalues and the plots of the model below. It appears that DAGMC does expect this group to contain a volume, maybe specifically the graveyard volume.

Original groups:

[Group 1, Name: picked,
 Group 2, Name: mat:no-void fuel
 Volume IDs:
 [1 2],
 Group 0, Name: mat:Graveyard
 Volume IDs:
 [6],
 Group 0, Name: mat:clad
 Volume IDs:
 [3],
 Group 0, Name: temp:300
 Volume IDs:
 [2],
 Group -1, Name: mat:water_comp
 Volume IDs:
 [6]]

Original model image (water colored blue):

Modified model groups:

 Group 1, Name: mat:no-void fuel
 Volume IDs:
 [1 2],
 Group 2, Name: mat:clad
 Volume IDs:
 [3],
 Group 3, Name: temp:300
 Volume IDs:
 [2],
 Group 4, Name: mat:water_comp]

Modified model plot: