Cleaning up the get_indices function for HexLattice a bit.
Cleaning up the get_indices function for HexLattice a bit.

liangjg/openmc,liangjg/openmc,shikhar413/openmc,paulromano/openmc,mit-crpg/openmc,smharper/openmc,amandalund/openmc,walshjon/openmc,wbinventor/openmc,shikhar413/openmc,mit-crpg/openmc,amandalund/openmc,paulromano/openmc,walshjon/openmc,wbinventor/openmc,smharper/openmc,liangjg/openmc,liangjg/openmc,smharper/openmc,paulromano/openmc,wbinventor/openmc,walshjon/openmc,shikhar413/openmc,mit-crpg/openmc,amandalund/openmc,amandalund/openmc,walshjon/openmc,mit-crpg/openmc,shikhar413/openmc,paulromano/openmc,smharper/openmc,wbinventor/openmc