Concepts

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Upset and downset of concept object now are iterable-returning methods instead
of properties (backwards incompatible).

Changed concept object minimal generating properties to be computed on request
instead of being precomputed during lattice build; changed minimal and
attributes from properties to methods (backwards incompatible).

Improved unicode support.

Changed pickling of lattice and concepts.

Changed ordering of ``downset`` and ``lower_neighbors`` to longlex.

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Add context file ``'csv'`` format.

Added newline normalization to context file loading.

Update ``bitset`` dependency to 0.5 (better neighbors).

Backwards incompatible: removed underscore from (from|to)_(string|file) method
names.

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Update ``graphviz`` interface dependency to 0.2 (UTF-8 support) with changed api.

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Support empty iterables in ``Lattice.join()`` and ``.meet()``.

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Simplified ``graphviz`` quoting.

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Improved visualization.