Threedigrid

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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()``.

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- Introducing subset fields that can be used to query results that are collected
only for subsets of the model, like the 2D section.

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- Fix lookup_index functionality for composite fields.

- Make model name property optional. That is, 'unknown' will be returned if the
name cannot be derived.

- Changed Depth/width fields on breach-timeseries to breach_depth and breach_width.

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- ``_get_composite_meta()`` does not raise an AssertionError anymore if
composite field attributes differ. Instead a warning is issued.

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- Bug fix: ``threedicore_result_version`` must be a property.