Changes in demo app to take advantage of GCE enhancements.

This fixes #36 

The following commits complete the first check in the checklist of the issue #36 (this is the first time I use checklists, I would check it myself but I don't know if that's correct. Perhaps you'd like to review it first and check it when you're sure this completes that task. Otherwise let me know and I'll check it)

Sorry, perhaps I should have made this change in another PR, but it was the first thing I needed to start coding the changes to the demo app. The first commit of this PR also fixes #37 
Also, if you use `altitude = float(record[4]) if record[4] != "" else None`, then you can check for `None` instead of `""` later on, which is cleaner. Also you can skip the additional block under this one.