Pygeoprocessing

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* Fixing LICENSE.TXT to .txt issue that keeps reoccurring.

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* Fixed an issue where int32 dems with INT_MIN as the nodata value were being
treated as real DEM values because of an internal cast to a float for the
nodata type, but a cast to double for the DEM values.
* Fixed an issue where flat regions, such as reservoirs, that could only drain
off the edge of the DEM now correctly drain as opposed to having undefined
flow directions.

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* Fixed a memory issue for DEMs on the order of 25k X 25k, still may have
issues with larger DEMs.

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* Fixed an issue so tox correctly executes on the repository.
* Created a history file to document current and previous releases.
* Created an informative README.rst.

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* Fixing issue that caused "LICENSE.TXT not found" during pip install.

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* Fixing issue with automatic versioning scheme.