add white boundary in surface.py
add white boundary in surface.py

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