Added russian_roulette subroutine.
Added russian_roulette subroutine.

nhorelik/openmc,sxds/opemmc,liangjg/openmc,liangjg/openmc,smharper/openmc,samuelshaner/openmc,kellyrowland/openmc,liangjg/openmc,shikhar413/openmc,walshjon/openmc,samuelshaner/openmc,mit-crpg/openmc,lilulu/openmc,wbinventor/openmc,liangjg/openmc,amandalund/openmc,bhermanmit/cdash,paulromano/openmc,walshjon/openmc,johnnyliu27/openmc,smharper/openmc,johnnyliu27/openmc,lilulu/openmc,mit-crpg/openmc,shikhar413/openmc,shenqicang/openmc,paulromano/openmc,johnnyliu27/openmc,smharper/openmc,wbinventor/openmc,mit-crpg/openmc,johnnyliu27/openmc,paulromano/openmc,mjlong/openmc,samuelshaner/openmc,mit-crpg/openmc,amandalund/openmc,paulromano/openmc,shenqicang/openmc,walshjon/openmc,kellyrowland/openmc,shikhar413/openmc,nhorelik/openmc,smharper/openmc,walshjon/openmc,amandalund/openmc,lilulu/openmc,shikhar413/openmc,mjlong/openmc,bhermanmit/openmc,bhermanmit/openmc,samuelshaner/openmc,sxds/opemmc,amandalund/openmc,wbinventor/openmc,wbinventor/openmc