To avoid future mistakes when overriding functions, this commit suggests
to use the `override` keyword as suggested by clang-tidy.

How this commit was produced:

1. Compile with CMake, exporting the compile commands as described here:
   https://stackoverflow.com/questions/20059670/how-to-use-cmake-export-compile-commands

2. Build

3. Run this in the build directory:
   ```
   run-clang-tidy -header-filter="interface/.*" -config-file=../.clang-tidy -export-fixes
 fixes.yaml . -j20
   ```

4. Copy the `fixes.yaml` back to the main repo and apply fixes with:
   ```
   clang-apply-replacements .
   ```

5. Replace `ClassDef` macros with `ClassDefOverride` to be consistent.

Follows up on `91b791a`, making sure that only the right overload is
implemented.

Also, apply the same fix to the `CachingAddNLL` class.