a) Implemented energy threshold on cells to be assigned to clusters

a) Implemented energy threshold on cells to be assigned to clusters
   In order to be fully consistent (ie before this gets ported to the
   release) the rec param should be updated to be zero.
   (it is 10 GeV right now the code used a hard coded value of zero)

b) Implemented switch (default off) to also use the energy gradient
   to decide whether a cell is considered to be neighbor or not.
   At the moment I am only playing with this option so it is not
   interfaced with the rec param.