reads and displays the battery level

The commits in this branch treat the battery level as another sensor value that is sent to Field Day with each sample. Merge corresponds to #6 (closed) , with implications for platforms#2.

These are the changes I've made to each component in my test environment:

• Field Day: read and display battery level
• platforms: send battery value as one of n sensor values
• database: add to fieldday_sensor a battery sensor for each relevant platform; I've so far done this only for Maria

I've tested the code and it works, but I've marked this as WIP to ask: Are we comfortable treating the battery as a sensor?

If so, I will (in this order):

• generalize this approach to all elevation platforms
• update the production database with each of the new sensor types
• push my changes to the elevation platform
• clean up the Field Day code on this branch
• merge this branch
• test
• when all is working, close #6 (closed) and platforms#2

If we're not comfortable treating battery level as a sensor, we can rethink.