Processor imu2d

The 2D IMU case has been implemented and works. Work now begins on implementing the slanted plane case, but this branch can be merged already

demos/processor_imu2d.yaml

+type: "ProcessorImu2d"              # This must match the KEY used in the SensorFactory. Otherwise it is an error.
+
+time_tolerance: 0.0025         # Time tolerance for joining KFs
+unmeasured_perturbation_std: 0.00001
+
+keyframe_vote:
+    voting_active:      false
+    voting_aux_active:  false
+    max_time_span:      2.0   # seconds
+    max_buff_length:  20000    # motion deltas
+    dist_traveled:      2.0   # meters
+    angle_turned:       0.2   # radians (1 rad approx 57 deg, approx 60 deg)