IMU sensors: add sensorName tag in FT IMU xml

[martinaxgloria]

This PR should partially solve https://github.com/robotology/robots-configuration/issues/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 https://github.com/robotology/robots-configuration/issues/377, I modified the nws attach level to 10, while previously was set at 15, the same level as the detach phase.