Preprocessing functions should leave additional columns untouched and potentially support magnetometers directly

[AKuederle]

At the moment preprocessing functions for rotation and coordinate transformation only rotate acc and gyr data.

All other columns are actually removed from the dataset. While we don't have any algorithms that support anything besides acc and gyr data at the moment, it would be nice to allow users that have additional columns (i.e. mag or baro) to use the pre-processing functions without any additional work.

In particular the magnetometer would require special attention, as we would need to define a sensor and a body frame for it and explicitly rotate it as part of the respective functions.

This was first suggested in #50