refactoring bounding_box and cleaning up constructors
refactoring bounding_box and cleaning up constructors

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