Add script to run clang-tidy on a source directory

The script allows to run clang-tidy on the complete project by passing the path
to the CbmRoot source directory or only a subdirectory by passing the path to
the subdirectory.
Some of the compilation units should not be tested by clang-tidy.
Exclude checking the dictionaries G__*.cxx since they generated by root-cling.
Exclude all targets from the external directory. This isn't our code and must
not be checked by us.