Cnftools

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- Unified command-line utilities into ``cnf``
- Added ``cnf stats ...``
- Added ``cnf simplify ...``
- Expanded ``cnf karps21``
- Added ``cnftools.simplify`` module
- Expanded ``cnftools.karps21`` module
- Started improving documentation
- Added unit tests

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December 2, 2018

Initial release

- Utility functions
- Saving and loading Dimacs CNF files
- Tseytin transformer
- Command-line utilities
- ``to3cnf``
- ``kcoloring``