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]