Feature request. Make void / vacuum material to use with DAGMC

[jon-proximafusion]

Description

When working with a DAGMC file it has predefined material tags
It would be useful I we can make a VOID material in OpenMC with no density and no nuclides so that we can "switch off" materials.

Alternatives

Exploit the vacuum bug in DAGMC by setting the material name to include the word 'vacuum' svalinn/DAGMC#805 which is being fixed soon

Compatibility

This would allow empty materials to be made without error, this is currently not possible

[pshriwise]

Good call @shimwell!

This would potentially be useful on all geometry types yeah? I think an attribute on the openmc.Settings class (possibly names void_geometry) might be the way to go.

[shimwell]

when trying this I found it natural to make an empty openmc material and name it after the dagmc material tag

[pshriwise]

I think that would will be possible in an even more general way after #3056 is merged using the material assignment overrides included there.