CombinedImuFactor covariance issue

[Kostia Antoniuk]

Hey Christian,

Please, correct me if I am wrong, but it seems that CombinedImuFactor uses wrong covariance matrix, i.e. if we have frame_i and frame_j CombinedImuFactor evaluates error in the frame_j while the covariance matrix seems to be expressed in the frame_i, which is not correct as factor will be unwhitened with the wrong covariance.

Best, Kostia.