This is essentially a heavily modified version of
AsgxAODNTupleMakerAlg for the specific purpose at hand, i.e. writing
out MET variables as `float` instead of `std::vector<float>`.

There is probably some overlap of code with the original algorithm,
but I'd rather clean that up at a later point, as I anyways have to
revisit how I handle the output n-tuples in a better way.