Threedi-modelchecker

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- Add check `ConnectionNodesDistance` which ensure all connection_nodes have a minimum
distance between each other.
- Set the geometry of the following tables as required: impervious_surface, obstacle,
cross_section_location, connection_nodes, grid_refinement, surface,
2d_boundary_conditions and 2d_lateral.
- Add check for open cross-section when NumericalSettings.use_of_nested_newton is
turned off.
- Add checks to ensure some of the fields in numericalSettings are larger than 0.
- Add check to ensure an isolated pipe always has a storage area.
- Add check to see if a connection_node is connected to an artifact
(pipe/channel/culvert/weir/pumpstation/orifice).

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- Changed Pipe.calculation_type to include broad- and shortcrest.

- Bugfix: Pumpstation.lower_stop_level should be higher than
models.Manhole.bottom_level.

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- Bugfix: made the `ConnectionNodesLength` backwards compatible with sqlalchemy 1.1.

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- Added `ConnectionNodesLength` check to check the length between a start- and end node
is above a certain threshold. Configured this check for pipes, weirs and orifices.

- Configured checks to see if the length of a linestring geometry is larger than 0.05m
for culverts and channels.

- Chaned GlobalSettings.start_date and GlobalSetting.start_time into type Text and
added two checks to see if the fields are valid datetime and date respectively.

- Configured extra check: use_1d_flow must be set to True when your model has 1d
elements.

- Removed `ConditionalCheck` and replaced it with `QueryCheck`.

- Added type-hinting.

- Created `CustomEnum` for `Enum` objects.

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- Fixed some misconfigured checks, see https://github.com/nens/threedi-modelchecker/issues/10.

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- Set language of travis to python and test for python 3.6 and 3.7.

- Update to following columns to be non-nullable: Levee.the_geom,
Culvert.invert_level_start_point and Culvert.invert_level_end_point.

- Removed threedigrid from requirements.

- Configured extra checks: Pumpstation.lower_stop_level > Manhole.bottom_level.

- Configured extra checks: Pipe.invert_level >= .Manhole.bottom_level.

- Added additional check type: QueryCheck.