Change depthCor* static members to static std::atomic<> members for thread safety.
These parameters are set once in the PFCluster constructor from parameter set
or set once to default values in PFCluster.cc. After that they are only read.
Merge pull request #2664 from gartung/statics-DataFormats-ParticleFlowReco

Thread Safety  DataFormats/ParticleFlowReco -- Change depthCor* static members to static std::atomic