Enhancement: parallelization of UniqueList 

I suggest adding these two functions to the `unique_list.py` file.
It will make the UniqueList picklable and thus enable parallelization, e.g., with multiprocessing or concurrent.futures.

```
def __reduce__(self):
        return (self.__class__, (list(self),))

def __setstate__(self, state):
    self._set = set(state)
    super().__init__(state)
```