covariance matrix sizes incompatibility in 2d case

Issue

subscriber_imu.cpp crashes in line 79, when we have an IMU2D sensor. The IMU3D case works fine.

Possible Cause:

In line 79 we call computeNoiseCov() which returns a Eigen::Matrix3d (wolf_lib->plugins->imu, include/imu/sensor/sensor_imu.h line 96)

Then we assign this Eigen::Matrix3d to cov, which was defined as Eigen::Matrix6d (subscriber_imu.cpp, line 79)