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Enhanced, maybe useful, data containers. For now, a versioned dict and an ordered versioned dict

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# Extra Dictionary classes and utilities for Python

New utilities to be added as they are devised

## VersionDict

A Python Mutable Mapping Container (dictionary :-) ) that
can "remember" previous values.
Use it wherever you would use a dict - at each
key change or update, it's `version` attribute
is increased by one.

### Special and modified methods:

`.get` method is modified to receive an optional
named `version` parameter that allows one to retrieve
for a key the value it contained at that respective version.
NB. When using the `version` parameter, `get` will raise
a KeyError if the key does not exist for that version and
no default value is specified.

`.copy(version=None)`: yields a copy of the current dictonary at that version, with history preserved
(if version is not given, the current version is used)

`.freeze(version=None)` yields a snapshot of the versionDict in the form of a plain dictionary for
the specified version

### Implementation:
It works by internally keeping a list of (named)tuples with
(version, value) for each key.

### Example:


>>> from extradict import VersionDict
>>> a = VersionDict(b=0)
>>> a["b"] = 1
>>> a["b"]
>>> a.get("b", version=0)

For extra examples, check the "tests" directory

## OrderedVersionDict

Inherits from VersionDict, but preserves and retrieves key
insertion order. Unlike a plain "collections.OrderedDict",
however, whenever a key's value is updated, it is moved
last on the dictionary order.

### Example:
>>> from collections import OrderedDict
>>> a = OrderedDict((("a", 1), ("b", 2), ("c", 3)))
>>> list(a.keys())
>>> ['a', 'b', 'c']
>>> a["a"] = 3
>>> list(a.keys())
>>> ['a', 'b', 'c']

>>> from extradict import OrderedVersionDict
>>> a = OrderedVersionDict((("a", 1), ("b", 2), ("c", 3)))
>>> list(a.keys())
['a', 'b', 'c']
>>> a["a"] = 3
>>> list(a.keys())
['b', 'c', 'a']

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