Sub: avoid forcibly setting altimeter measurement noise on boot

[ES-Alexander]

Bug report

Issue details

ArduSub automatically sets the altimeter measurement noise on boot, which means setting it with a parameter does not persist across reboots.

Version master, back to 5+ years ago.

Platform [ ] All [ ] AntennaTracker [ ] Copter [ ] Plane [ ] Rover [X] Submarine

[ES-Alexander]

This is based on quite weird naming of things, and some tenuous logic. It should be possible to set the measurement noise without adjusting the parameter (to avoid persistent changes that may not match the user's hardware configuration), and to use the parameter otherwise (with a reasonable default value).