Adding VTK image format to ignore.
Adding VTK image format to ignore.

smharper/openmc,amandalund/openmc,walshjon/openmc,liangjg/openmc,mit-crpg/openmc,liangjg/openmc,shikhar413/openmc,amandalund/openmc,shikhar413/openmc,walshjon/openmc,paulromano/openmc,shikhar413/openmc,wbinventor/openmc,wbinventor/openmc,johnnyliu27/openmc,liangjg/openmc,wbinventor/openmc,liangjg/openmc,walshjon/openmc,paulromano/openmc,walshjon/openmc,smharper/openmc,smharper/openmc,amandalund/openmc,paulromano/openmc,mit-crpg/openmc,paulromano/openmc,wbinventor/openmc,mit-crpg/openmc,amandalund/openmc,johnnyliu27/openmc,mit-crpg/openmc,smharper/openmc,shikhar413/openmc,johnnyliu27/openmc,johnnyliu27/openmc