[BUG] DataPanel returns dict for _state_keys, not set

[gtmtg]

Describe the bug
DataPanel's default return type for _state_keys is an empty dict, not an empty set. This causes a crash for subclasses of DataPanel that attempt to union their _state_keys with super()'s _state_keys.

File ~/Dropbox/Projects/actively/lib/src/actively/datasets/columns.py:54, in ActivelyDataset._state_keys(cls)
     52 @classmethod
     53 def _state_keys(cls) -> Set[str]:
---> 54     return super(ActivelyDataset, cls)._state_keys().union({"name"})

AttributeError: 'dict' object has no attribute 'union'

[seyuboglu]

#224 just got merged into dev which should fix this