Added is_valid method to BoundingBox.
Added is_valid method to BoundingBox.

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