Threedigrid

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- Fixed `_field_model_dict` being a class variable.

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- Added sumax to nodes

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- Bug fix in `h5py_file` method mapping.

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- Added sources and sinks (q_sss) to threedigrid.

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- Manholes preparation fixed mapping in ``connection_node_pk``.

- Added `to_structured_array` method for retrieving (filtered) results
as Numpy structured array instead of an OrderedDict

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- Set fixed type to the fields `code`, `display_name` and `shape`. These fields
now have a fixed lenght of 32, 64 and 4 characters respectively.