[SDFormat->MJCF] Identify a small set of commonly used sensors

[azeey]

This table shows the mapping of sensors from SDFormat to MJCF.	SDFormat Sensor	Mapped MJCF Sensor
air_pressure
altimeter	framepos
camera	body/camera
contact	sensor/touch
boundingbox_camera
boundingbox
custom
depth_camera
depth
force_torque	sensor/force and sensor/torque
gps
ray/gpu_ray/lidar/gpu_lidar	sensor/rangefinder if sample is 1
imu	sensor/accelerometer and sensor/gyro
logical_camera
magnetometer	sensor/magnetometer
multicamera	body/camera
navsat
rfid
rfidtag
rgbd / rgbd_camera
segmentation/ segmentation_camera
sonar
thermal/ thermal_camera
wireless_receiver
wireless_transmitter

Given this mapping and the relative frequency of use of this sensors, I propose we map the following SDFormat sensors for our initial implementation:

• imu
• force_torque
• camera
• framepos