code: function (stemG, wsVI, laVI, SLA) {
      if (stemG < 10) {
        stemG=10;
      }
      var VI = Math.pow( (stemG / wsVI.stems_per_stump / wsVI.constant),(1 / wsVI.power) );
  
      // Add up for all stems
      var la = laVI.constant * Math.pow(VI,laVI.power) * wsVI.stems_per_stump;
      var wf = 1000 * (la / SLA);  // Foilage Weight in g;
      var pfs = wf/stemG;
      return pfs;
    }
docs: Calculates the pfs based on stem with in G.  Uses volume Index as guide