Implement getter for Cell.atoms
Implement getter for Cell.atoms

Make type checking in Cell.volume more compact

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