Further Power-Saving Improvements

[darksidelemm]

Just leaving myself some notes:

• Gap between transmissions, send 'pips' every X seconds when in this gap so listeners know there's something there.
• Look at more GPS power-saving settings.
• Power down radio more between pips?

[darksidelemm]

A lot of this has now been implemented:

• uBlox GPS power-save mode is working reliably.
• A 'deep sleep' mode that powers down the GPS has been implemented.

There's still possibly some improvements to be made in putting the STM32 into a lower-power sleep state. At the moment it's essentially busy-waiting, running at full speed. There may also be other devices on the board drawing current which we can disable in software.