Features

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Switch ``setup.py`` dependencies to version ranges.

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Easier customization.

Improved documentation.

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Added ``add_config()``.

Added ``make_features()``.

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Added ``orthogonal_to()``.

Rename ``unifcation`()`` to ``union()``.

Improved doctests.

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Update ``concepts`` dependency to 0.5 and improve separation of concerns.

Changed ``upset`` and ``downset`` from properties to methods (backwards incompatible).

Order downsets longlex instead of shortlex.

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Update ``concepts`` dependency to 0.4.