Finished the derivation of the free gas scattering kinematics.
Finished the derivation of the free gas scattering kinematics.

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