[Feature request] - Support for OpenMC lattice feature

[jeinstei]

Is your feature request related to a problem? Please describe.

Currently openmc XML input does not support lattice objects openmc lattice example

Describe the solution you'd like

Update XMLinput.xml to support <lattice> input keys by mapping them to a number of surfaces.

Describe alternatives you've considered

A new internal representation of a lattice type to minimize the creation of new cells/surfaces/objects. This would allow for utilizing solid_replica STEP keys and minimizing the risk of internal data transformations. A challenge would exist in handling material transformations.

Additional context

I am willing to look into this and PR it, I believe, but given the possible solutions, discussion on best path forward would probably make sense first.

[jeinstei]

Looks to be the same as #28 , essentially. I might take a look at this in terms of what is needed for a PR myself, though