hidden/fixed "missing override" warnings exposed after !192

- warnings from external headers are hidden declaring the include directories as `-system`
- warnings from a couple of new files have been fixed
- warnings in `HistogramSvc` are hidden because unavoidable (see f83c3d8e)
- warnings related to CLHEP-136 have been hidden with a special trick (see 0a238135)

See merge request !205