Added description of universes and lattices.
Added description of universes and lattices.

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