Lattice outer feature for OpenMC compatibility

[wbinventor]

The OpenMC code includes an outer attribute for the Lattice class (link). At the time of this writing, is the only geometric modeling option in OpenMC which is not supported in OpenMOC. There is no reason the OpenMOC codebase cannot support an outer attribute on the Lattice class, and it should be readily implementable if so desired. However I no longer have the time to do so, but want to document this minor shortcoming here for posterity.

[ternence6]

agree.OpenMOC also can't deal with the Hexagonal Lattices from geometry made by OpenMC.