Change Datastream/observationType to enum and check its values

[hylkevds]

Currently any String is allowed, but only one of the official codes should be allowed:

• OM_Observation: Observation;
• OM_Measurement: Measurement;
• OM_CategoryObservation: Category Observation;
• OM_CountObservation: Count Observation;
• OM_TruthObservation: Truth Observation;
• OM_TemporalObservation: Temporal Observation;
• OM_GeometryObservation: Geometry Observation;
• OM_ComplexObservation: Complex Observation;
• OM_DiscreteCoverageObservation: Discrete CoverageObservation;
• OM_PointCoverageObservation: Point Coverage Observation;
• OM_TimeSeriesObservation: Time Series Observation.