Fix cpplint errors

Fixes errors from the following categories:

- `readability/inheritance`
- `readability/braces`
- `readability/multiline_comment`
- `whitespace/newline`
- `whitespace/empty_if_body`
- `build/include_what_you_use`
- `build/header_guard`

Author: vkucera

Tutorials/ML/applyMlSelection.cxx

@@ -36,6 +36,7 @@
 
 #include <cstdint>
 #include <cstdlib>
+#include <string>
 #include <vector>
 
 using namespace o2;

Tutorials/src/ConfigurableCutLinkDef.h

@@ -9,6 +9,8 @@
 // granted to it by virtue of its status as an Intergovernmental Organization
 // or submit itself to any jurisdiction.
 
+// NOLINT(build/header_guard)
+
 #pragma link off all globals;
 #pragma link off all classes;
 #pragma link off all functions;

Tutorials/src/associatedExample.cxx

@@ -150,7 +150,7 @@ struct PartitionColExtra {
 struct BuildHmpidIndex {
   // build the index table HMPIDTracksIndex
   Builds<aod::HMPIDTracksIndex> idx;
-  void init(InitContext const&) {};
+  void init(InitContext const&) {}
 };
 
 struct ConsumeHmpidIndex {

Tutorials/src/preprocess.cxx

@@ -28,6 +28,7 @@
 
 #include <algorithm>
 #include <cstdlib>
+#include <string>
 
 using namespace o2;
 using namespace o2::framework;

Tutorials/src/reweighting.cxx

@@ -51,6 +51,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <memory>
+#include <string>
 #include <vector>
 
 using namespace o2;