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.

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- Bug fix: dict.values() and dict.keys() in python 3 are causing some
unintended behaviour.

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- New release due to failing uploads.

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- Add aggregation option 'current' to volume and intercepted_volume.

- Using a non-tuple sequence for multidimensional indexing is deprecated; use
`arr[tuple(seq)]` instead of `arr[seq]`.

- Properties should be strings so we can use string methods on them.

- Do not prepare levees if there aren't any.

- Split requirements file to allow for finer grained builds (for instance to
generate the documentation).

- Add 'intercepted_volume' to NodesAggregateResultsMixin.

- Split requirements file to allow for finer grained builds (for instance to
generate the documentation).

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- Added BooleanArrayField for boolean values and use it for `is_manhole` filter.
NO_DATA_VALUE is interpreted as False.