[GPS] Add power down mode to GPS

[TD-er]

See example code (and great analysis) here: https://github.com/StuartsProjects/GPSTutorial

N.B. some modules from other brands still use uBlox modules, like the BN220, which apparently uses uBlox M8.

ToDo:

• [x] Implement call to sleep mode
• [ ] Add GPIO pin to toggle power on GPS module
• [ ] Add timeout to get a fix before letting the unit sleep again
• [ ] (optional) Select interested fields before going to sleep again (e.g. date/time, position, confidence level on accuracy of data)
• [ ] (optional) Use uBlox specific Power Save Mode configuration. (uBlox 8 manual chapter 13.2.2 Configuration)
• [ ] Add sleep time setting for sleep mode GPS (uBlox only?)
• [ ] Allow uBlox "Super-Efficient Mode" (depending on protocol version)
• [ ] Allow setting baudrate (and maybe also setting it in the GPS module)

[TD-er]

Initially #3849 was meant to implement all of this, but sadly sending commands to a GPS module isn't as simple as it seems. Apparently the receiving module may sometimes be so chatty that it corrupts its own receive buffer and thus signalling invalid messages. After 100 of those, it simply mutes its own RX (luckily this is sent in a TXT NMEA message) for a while.

So this issue is still not complete.

[TD-er]

Another option is to create a separate u-blox mode and then using this library: https://github.com/sparkfun/SparkFun_u-blox_GNSS_Arduino_Library