Threedigrid

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- Added `content_pk` to the export_constants of all structures which have a
`content_pk`.

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- An empy array [] is returned now instead of None if there is no
dataset.

- Line geojson items need to use line_geometry values

- Bugfix for `Model._get_subset_idx` not instantiating new subsets with their parent's
mixins.

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- Use 'ga.xxx.id.size' to check if certain submodels (like channels/weirs/manholes)
are available for geojson exports.

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- Allow older pyrpoj versions. (pre 2.2.0)

- Bugfix for timeseries start_time=0 selection and allow indexes=slice(x,x,x)
in combination with sample() method.

- Added try-except surrounding all imports of the package `geojson`. This package is
only available when threedigrid is installed with the extra [geo] extension.

- Add `ORIFICES_EXPORT_FIELDS` to export_constants.

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- Add extra field `pixel_width` to cells

- Bugfix for GeometryIntersectionFilter: filter was only checking on
intersecting bounding boxes

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- The `sample` method needs to skip the last timestamp for SWMR
to work correctly. (time dataset can have one item more
than datasets with timeseries)