Extend and complete implementation of new DataStreamMET4FOF

[BjoernLudwigPTB]

The class DataStreamMET4FOF was refined and extended quite a bit regarding its streaming capabilities through a provided generator function and regarding the incorporation of our package time-series-metadata.

We should now extend especially the metadata idea. The idea would be:

• [ ] integrate the metadata storing into the constructor
• [ ] make metadata mandatory but allow for intentionally leaving it out
• [ ] provide proper setter and getter, partly resolve in #158 by adding a getter, if we want to leave it with set_metadata this is done
• [x] provide docstrings for all parts of the public interface to make them appear on ReadTheDocs as well, resolved in #158
• [ ] maybe more?!