Added ROOT6 compatibility
Remove mutable keyword by removing const declaration from unused function
Merge pull request #1389 from dinardo/ROOT6-Compatible-Pixel-BeamMonitor

ROOT6 -- Added ROOT6 compatibility to Pixel/BeamMonitor
Merge pull request #1415 from wmtan/RemoveMutableKeywordFromHiGenData

Multithreading fixes -- Remove mutable keyword by removing const declaration from unused function