Add TideGaugeLocationLink model

This allows us to link a TideGauge to a Location such that
RawMeasurements will be processed and added as Observations to the
linked Location.

[#83030886]
Add tests for RawMeasurement->Observation creation

Test that if there's a link between a TideGauge and a Location then
RawMeasurements should result in Observations being created.

[#83030886]
Implement RawMeasurement => Observation creation

When a RawMeasurement is created or updated, check to see if there's a
linked Location and, if so, create/update an Observation at that
Location.

This allows us to designate a particular TideGauge as the source of
Observations for a broader Location (eg Gladstone dock, where there are
multiple gauges)

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

[Delivers #83030886]
Add tests for TideGauge.linked_location property
Merge pull request #115 from sealevelresearch/tide-gauge-location-link

Create Observation from RawMeasurement