Partitions are not pickle-able

[zschutzman]

partition objects cannot be passed through the pickle serialization library. Below are more details and a fix. If we want to implement it, I can make a PR in the near future. If not, someone who really wants to pickle their partitions can look at this for guidance.

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I tried to pickle a partition object and was met with

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "home/gerrychain/partition/partition.py", line 115, in __getattr__
    _k = self.__getitem__(key)
  File "/home/gerrychain/gerrychain/partition/partition.py", line 109, in __getitem__
    self._cache[key] = self.updaters[key](self)
KeyError: '__getstate__'

and while I don't understand the inner workings of the pickle library, I did manage to resolve the problem by redefining Partition.__getitem__ and Partition.__getattr__ to the following:

def __getitem__(self, key):
    """Allows accessing the values of updaters computed for this
    Partition instance.

    :param key: Property to access.
    """
    try:
        if key not in self._cache:
            self._cache[key] = self.updaters[key](self)
        return self._cache[key]
    except:
        return None

def __getattr__(self, key):
    _k = self.__getitem__(key)
    if _k == None:
        raise AttributeError
    return _k