Feature: Support Hex Lattices

Currently Hex lattices are not supported by this adapter though they are supported by both MCNP and OpenMC. I suspect to implement this would require modifying [this source](https://github.com/openmc-dev/openmc_mcnp_adapter/blob/main/src/openmc_mcnp_adapter/openmc_conversion.py#L668). I may look into to implementing the minutia of this myself. 