iCub3: add realsense imu

[Nicogene]

This PR adds the imu sensor contained in the intel realsense of the chest (D435i).

The sensor has been called simply chest_imu_acc_1x1 similarly to the one in the head (head_imu_acc_1x1).

The name can be changed/discussed.

As explained here IntelRealSense/realsense-ros#927, the imu measurement are referred to the depth frame, and here it is what I read from the /icubSim/chest/inertials/measures:o:

(((-0.0803231336812528090574 0.105600261803569975272 -0.0575165845263962208223) 39.9040000000000034674)) (((0.0139543972809083767495 -9.89029974843219328307 -0.000291674178350950774607) 39.9040000000000034674)) (((0.0 0.0 0.0) 39.9040000000000034674)) (((-0.0392458976633651424049 0.00484875015179548479738 0.00352405535083964369103) 39.9040000000000034674)) () () () () () ()

As you can see, on the y we measure the g (-9.8) since in the depth frame the y points downwards.

Please review code.

cc @S-Dafarra @prashanthr05 @pattacini

[traversaro]

I guess we have other IMUs in the chest (i.e. in the EMS), so eventually we may want to discuss the naming of the sensor, but this does not block exposing this IMU.

[prashanthr05]

We can cross-verify the frames using rs2_extrinsics right? This function gives us the transforms between each senor mounted in the realsense hardware.

[Nicogene]

We can cross-verify the frames using rs2_extrinsics right? This function gives us the transforms between each senor mounted in the realsense hardware.

For sure between optical sensors, but I have no idea about the imu, I never used the D435i.

In any case, the rs2_extrinsics should give you the transformations between the physical sensors, I don't expect that the imu is physically where is placed the depth sensor, but I expect that internally it does the math in order to express the imu measurements in the depth frame.