Follow-up from "Draft: Add Wetterdienst docs"

The following discussion from !355 (merged) should be addressed:

• @Balzer_J started a discussion:

  This is also a future To-Do. The return type of from_node() is either a Connection or a dictionary of Connections. So when we want to call e.g. read_series() mypy doesn't know whether we have a single connection or a dictionary of connections. I would propose to split the functionality of from_node() into from_node() and from_nodes(). Then we would have type safety.