Update swagger required fields

[PietrH]

The swagger API reports no required fields for the observation model: https://uat-db.vespawatch.be/swagger/?format=openapi

definitions > observation > required

However, when importing a minimal record I get an error about required fields:

Failed to import observations: {'errors': [{'record': 1, 'error': 'Missing required fields for new record: observation_datetime, longitude, latitude'}]}

Please update the swagger docs to reflect this.