Gen particle pruner bug fix

Hi, 

Daughters of the particle with index 0 were not being assigned a mother because of the (idx > 0) requirement.  Changing this to (idx >= 0) fixes the problem so that all daughters are assigned a mother.  I've tested it out on a few samples and encountered no problems.  Please let me know (w.wulsin@cern.ch) if you have any questions.

Wells