Fix particleFlowClusterECAL timing.

@bendavid @slava77

Down to 0.05 sec/event from 0.12 sec/event in https://github.com/cms-sw/cmssw/pull/706.
The timing was 0.03 sec/event originally. 
Further improvements would require factorization of PositionCalc code to take advantage of the PFCluster's refs to its rechits. Most of the extra time is spent building the necessary structures for pos calc and then in poscalc it is spent searching for the rechits in the sorted collection.

Since the PFClusters have the refs to the rechits that compose them, searching for the associated rechit again isn't really necessary. 

@argiro : Perhaps we could factor apart the rechit energy determination and position weighting parts of PositionCalc into two steps?