applied some suggestions from code review
applied some suggestions from code review

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