Add unit test for CellInstanceFilter
Add unit test for CellInstanceFilter

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