Threedi-modelchecker

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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.

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- Fix setup.py.

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- Added missing NotNullChecks to the config.py

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- Retry release (release of 0.4 is missing changes).

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- Update to readme.
- No longer raise a MigrationTooHighError when the migration is larger than expected.