Change IBeamIntensity API to take run and event number explicitly

• The IBeamIntensity API now takes the run and event number explicitly.
• StepArrayBM and ArrayBM now use ContextSpecificData for the t0 offset. SlotSpecificObj is not used because this also needs to work when there is no valid context
• ArrayBM now has a Seed property. If this is set to a non-zero value, it uses FastReseededPRNG seeded with this value and the run and event number instead of the AthRNGSvc
• FastReseededPRNG has been added to CxxUtils. For now xxhash is vendored, until it can be added to externals or the LCG release