Mark fission heating as redundant cross section
Mark fission heating as redundant cross section

Fission heating should not contribute to total cross section,
and marking the reaction as redundant ensures this. MT=318 for
fission heating is already included in the redundant reactions
to keep when exported to HDF5

Related to #1340

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