Bangle GPS Power management

Ok, so quick testing: Normally, Bangle.js draws 33-37mA with the GPS on.

With just UBX_CFG_INTERVAL(30,5);, I see it drop to 8mA for just a few seconds, but then it never drops that low again and stays at around 31mA.

However just running UBX_CFG_PMS() (so super-e 1 Hz) without anything else drops power usage to around 17mA. Power usage does spike sometimes to 33mA for a few seconds (I guess as it updates the fix) but it seems pretty good. There's also no need to call _SAVE for it to take effect - but obviously if not it'd need resetting each time.

Looking at https://www.u-blox.com/sites/default/fil­es/EVK-M8BZOE_UserGuide_%28UBX-17053592%­29.pdf

For lowest power consumption, Super-E mode supports continuous tracking with update periods up
to 10 s. To set update periods longer than 1 s, select default Super-E mode (1 Hz) and use Extended
Power Management message UBX-CFG-PM2 to set the update period.

So that could be an option I guess?