Mag data unit inconsistency in documentation

[dawnclaude]

According to the documentation, the EKF class constructor takes mag_data in mT units, but the update function takes mag_data in uT units.

[Mario Garcia]

Thanks. Both were actually wrong. Some manufacturers use uT, and some others mT, but the geomagnetic data is normally handled as nT by the National Agencies.

Fixed with commit a4e225b