WIP: slanted imu2d

We'll now extend the capabilities of the 2D IMU plugin to deal with slanted surfaces. The implementation should be relatively simple, using the Delta definitions of the 3D case with 2D objects, the developed Lie Theory of the 2D case, and the adapted equations of composition for the 3D case.

demos/sensor_imu2d.yaml

@@ -7,3 +7,4 @@ motion_variances:
     wb_initial_stdev:       0.350     # rad/sec - initial bias 
     ab_rate_stdev:          0.1       # m/s2/sqrt(s)           
     wb_rate_stdev:          0.0400    # rad/s/sqrt(s)
+    orthogonal_gravity:     true