Threedigrid

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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.

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- Added additional exporters for

- 2D_GROUNDWATER
- 2D_OPEN_WATER
- 2D_VERTICAL_INFILTRATION

- Added method ``get_model_instance_by_field_name`` to the
``GridH5ResultAdmin`` class. This makes it possible to do reverse lookups
in situations where you have a field name but do not know which model it
belongs to. N.B the field must be unique otherwise an ``IndexError`` will
be raised.

- Added property ``dt_timestamps`` to the timeseries_mixin module.

- The version number is added to the ``__init__`` file dynamically using the
``pkg_resources`` API.

- Timestamps of all timeseries fields are shown for aggregation results.

- Timestamps in the aggregation results are filtered when retrieving subsets of timeseries.

- Introducing the ModelMeta class. Its main purpose at this moment is to compute all
possible combinations of composite_fields and aggregation variables.

- Fixed return statement of method slice (in class Model) which now takes
``**new_class_kwargs``.

- Empty or missing datasets are now displayed as ``np.array(None)`` instead of
raising an error.

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- New release using twine 1.11.