The usage of std::binary_function is really old legacy to define the functions
used in STL algorithms like std::sort. std::binary_function was deprecated with
C++11 and removed from C++ with C++17. Obviously it is still supported by some
compilers since compilation errors only occured with newer Apple clang versions.

Switch to the modern implementation using lambdas in the STL algorithms.
This also increase code readability since the code is located at the same place
where it is used.

The change was confirmed to not introduce changes in the output by comparing
the produced TTrees before and after the commit in all produced ROOT files.