Add era5 examples as python scripts

[uvchik]

Add Python examples

• [x] Download era5 data (single location and bounding box)
• [x] process single location with windpowerlib
• [x] process single location with pvlib
• [x] process bounding box

[birgits]

Thank you @uvchik for picking up work on the feedinlib again and adding new functionality to the examples, especially the multiprocessing parts which are quite useful. However, I think we need to discuss whether we want to provide example notebooks or python files, because in my opinion having both leads to an unnecessary maintenance overhead. I prefer notebooks as they are an easy entry point for people who don't know python very well, they can be nicely included in the documentation and we can offer them on binder so people can use the feedinlib without having to install it, if they just want to try it out (btw, did you take out binder @uvchik?). Furthermore, they can be converted to python scripts if someone prefers them, so I don't see the advantage of having additional python scripts. What's your opinion on that @oemof/feedinlib people?