De-virtualize `level_merger`

[ssoelvsten]

With #744 we closed #727 by making the level_merger use dynamic dispatch, i.e. virtual functions, to allow taking inputs from different types of sources. Yet, in practice we always know the type of the respective inputs: a generator function, a diagram file, or with #444 a diagram root pointer.

Hence, it should be possible to derive the type of the readers at compile-time and store them in a std::tuple. This way, we would regain the ~2% performance decrease we got with #744 .