Remove dependence of sample_target_velocity on particle.
Remove dependence of sample_target_velocity on particle.

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