Add script to apply clang-tidy to all targets

apply_clang_tidy.sh

+# This script assumes that clang-tidy is installed and in the PATH.
+# In addition, clang-tidy needs a compilation database, so you have to run
+# cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=1 .
+# in your build directory and then copy or symlink the resulting
+# compile_commands.json to the source directory.
+
+# It is possible to run clang-tidy on the whole directory at once, using parallelism and all.
+# However, even with the run-clang-tidy.py script and -j1, the fixes are applied several
+# times to each header (once for each compilation unit in which the header is included),
+# resulting in messed up files. We thus simply run clang-tidy sequentially for each
+# compilation unit. This takes a long time, but since we only have to do this every
+# now and then and can do other stuff while waiting for the script, this should be fine.
+
+# Headerchecks
+BUILD_DIR=../build/clang-debug/dune-xt
+CLANG_TIDY_BINARY=clang-tidy
+for file in `find ${BUILD_DIR}/headercheck -name *.hh.cc`; do
+    ${CLANG_TIDY_BINARY} --config-file=.clang-tidy -fix ${file}
+done
+# Regular C++ files
+for file in `find dune/xt/ -name *.cc`; do
+    ${CLANG_TIDY_BINARY} --config-file=.clang-tidy -fix ${file}
+done
+# Python bindings
+for file in `find python/ -name *.cc`; do
+    ${CLANG_TIDY_BINARY} --config-file=.clang-tidy -fix ${file}
+done