Fix some more fastidious suggestions by clang-tidy
This MR is a preparation for moving to clang-tidy
shipped with LLVM version 9.0. Some additional warnings have been added, which are fixed here or - in one case - disabled.
The disabled check concerns memory ownership, where we don;t have any issues in our code but depend on ROOT which still heavily utilizes raw pointers and assumes ownership of the objects silently.