Add a helper class for accessing the same branch by name across several files

This MR adds the NtupleBranchAccessByName helper class, which can be used to access branches by name if they are not members of a MiniTree class or access via members is not desired.

Access in this way is faster than repeated calls to NtupleBranchMgr::getBranch / NtupleBranchMgr::operator(), due to a caching mechanism, while also being safe when switching back and forth between input files in multicore running.