Initializing yaw orientation from magnetometer reading.

- added magnetometer biases (mag_bias/x and mag_bias/y) for hard-iron compensation.
- add two debug topics: imu/rpy/raw and imu/rpy/filtered. imu/rpy/raw can be used for computing mag. biases. imu/rpy/filtered topic is for user readability only. 
- added an option for tilt compensation and initializing yaw orientation from magnetometer reading.
Don't pass roll / pitch / yaw by reference here, simply pass them by value (remove the `&`).

The reason for this is that `publishRawMsg()` doesn't (and shouldn't!) modify these params, so that should be reflected in the method signature. That leaves us two options: `const float&` or simply `float`, and for primitive data types like float, the latter seems simpler.