Implement CommonBandsWorkChain for FLEUR

[janssenhenning]

Adds the FleurCommonBandsWorkChain and input generator.

One note:

• I noticed while testing, that the engines input cannot be omitted as the get_builder() method will raise an error that engines.bands.code is missing. But the implementations I used as a reference seem to handle this case at least partially. Is this wanted behaviour?

[janssenhenning]

@bosonie @sphuber I, unfortunately, cannot request a review directly so I'm pinging here. I think to request a review I also need to be approved first.