Merge pull request #48 from sealevelresearch/add-ea-observations

Periodically import EA measurement data
Add 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.
Add observations module with `Observation` model

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

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

[Delivers #75972854]
Add convert_measurements to convert Measurements

Convert 15-minute EA measurements with Ordnance Datum into 1-minute
Observations based on Chart Datum for the location.

https://www.pivotaltracker.com/story/show/76127328

[Delivers #76127328]
Convert measurements every 10 minutes