Improve DataStreamMET4FOF to get closer to MetrologicalDataStreamMET4FOF

[BjoernLudwigPTB]

There is some implementation need for improving the class DataStreamMET4FOF. It is already capable of streaming by setting a generator function and even has a method set_metadata. We should put some effort in finalizing these capabilities in terms of getting closer to some kind of MetrologicalDataStreamMET4FOF in resolving Met4FoF#130.