Create Observation from RawMeasurement

If there's a link between a `TideGauge` and a Location then new / updated
`RawMeasurements` should result in `Observations` being created.

- Add `TideGaugeLocationLink` model
- Add tests for RawMeasurement->Observation creation
- Override `save()` method on `RawMeasurement`

https://www.pivotaltracker.com/story/show/83030886