Add observations module with `Observation` model

This represents an actual sea level measurement at a particular location
and a particular minute in time.

Part of this was adding minute_in_time library with `Minute` model

This automatically rounds a datetime to a single minute. It will allow
Observations and Predictions to share a common point in time.

https://www.pivotaltracker.com/story/show/75972854

[Delivers #75972854]