Added example of creating MOX to pincell example.
Added example of creating MOX to pincell example.

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