9 DOF kalman filter with state represented as a quaternion
based on this paper:
Bernal-Polo, P.; Martínez-Barberá, H. Kalman Filtering for Attitude Estimation with Quaternions and Concepts from Manifold Theory. Sensors 2019, 19, 149. https://doi.org/10.3390/s19010149
uses Eigen linear algebra library