Threedigrid

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- Patch for converting numpy strings/bytes to float for both python2/3.

- Dropped NetCDF library and replaced opening NetCDF files with h5py

- Bumped h5py to 2.8.0

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- Made threedigrid >= Python 3.5 compatible.

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- Bug fix for issue 44: use the method ``get_filtered_field_value()`` instead
of ``get_field_value()`` for the count property.

- Properly closes netcdf-file in ``GridH5ResultAdmin``.

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- Add export functions for 2D to the ``export_all()`` collection.

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- Do not use ``pkg_resources`` to determine the current version but use
zest_releaser to update the version string in threedigrid/init.py

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- Use the custom ``NumpyEncoder`` to convert specific numpy types to native
python types when calling ``(geo-)json.dumps()``.