IMU sensors: add sensorName tag in FT IMU xml

[martinaxgloria]

This PR should partially solve #636. It adds:

• sensorName tag for all the ergoCub/iCub that exposes the IMU data from the FT sensors;
• the sensor name will be of type <part>_ft_imu and the simulated models will be aligned accordingly;
• as per Document attach/detach levels used for different kind of devices #377, I modified the nws attach level to 10, while previously was set at 15, the same level as the detach phase.