This setup is used for toy_convect and for the XRB stuff.  The init
code takes an input file (one of the _param* files) to determine what
is going on.

-- _params.xrb_mixed.hi_dens.tall.CNO

   This is the _params file that is used for the xrb_mixed stuff

-- _params.xrb_mixed.hi_dens.tall2.CNO

   This version differs from the above one with more buffer beneath
   the convective layer