Use class attribute in capi.Filter.__new__ to avoid the need to define __new__ in subclasses.
Use class attribute in capi.Filter.__new__ to avoid the need to define __new__
in subclasses.

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