Updates to mirror changes in pixels_ type in plot.cpp.
Updates to mirror changes in pixels_ type in plot.cpp.

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