REFAC: remove defaults from default data handler
DataHandlerSingleStation sets a lot of defaults. This leads to the problem, that some settings are set by the data handler and not in an experiment setup (e.g. station_type is not given in the experiment setup, but is set by the data handler). It should be more clear, what defaults are used. Therefore, I suggest to remove some defaults and use Nones instead.
- investigate which parameters shouldn't be filled with a default value by the data handler
- create a list with these parameters
- apply refactoring