* clang_tidy.cmake: added clang-tidy-11 to list of executables to look for
* .clang-tidy: disabled warnings we are (currently) not interested in
* fixed clang-tidy warnings
* CMakeLists.txt: there is no REQUIRED option for include()
* added a "run-clang-tidy" target when the executable exists / always create compilation database / added .clang-tidy with all existing warnings have been disabled for now