Add ptrs() interface to View which returns std::vector<edm::Ptr<T>>

This is the first step in removing refVector and ptrVector interfaces from View in order to support containers with 'refs' which point to multiple containers.
Merge pull request #6245 from Dr15Jones/returnVecPtrFromView

Add ptrs() interface to View which returns std::vector<edm::Ptr<T>>
Merge remote branch 'origin/CMSSW_7_3_X' into CMSSW_7_0_X