Switch compass to use true north

[chrisduerr]

Currently the compass reads data straight from iio-sensor-proxy. While this is easy, it does only provide a heading towards the magnetic north, which is significantly different from true north in many highly populated regions (like Quebec for example).

While an approximate accuracy is generally sufficient, a correction for the magnetic declination based on GPS position is likely sensible. Using a reasonable modern model to approximate for this should improve compass readings in relation to map tile orientation.

See https://www.ncei.noaa.gov/products/world-magnetic-model (lists software implementations for the world magnetic model).