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
Merge pull request #50 from sealevelresearch/convert-ea-data-into-observations

Script to convert Measurements into Observations