migrate ConvVxSorter and TrackMatchSorter to C++17 compatible style (and simplify)

The class, std::binary_function, is removed in C++17, so remove its use in ConvVxSorter and TrackMatchSorter. At the same time, I thought that using a functor instead of a function was a bit overkill (adding complication for no gain) so I changed them to be functions. Then there was a design question: do I leave them in the header as inline functions, or do I move then to a .cpp file. I decided to merge them into EMConversionBuilder and EMTrackMatchBuilder, which are the classes that use them. Pinging @christos and @rnaranjo in case they have objections to the design choices.