Changed sampling of angle for fission to isotropic.
Changed sampling of angle for fission to isotropic.

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