Further simplifications in arithmetic.py
Further simplifications in arithmetic.py

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