iCub 2.7 models do not have the xsens IMU

[traversaro]

All iCub versions >= 2.7, < 3 include the xsens IMU in the chest (see https://github.com/icub-tech-iit/documentation/blob/5d919960f2d71e25d845b1eb4b899ed0c389ae43/docs/icub_versions/index.md#L17), but we only add it if we are generating the KIT_007 model (see https://github.com/robotology/icub-models-generator/blob/dbc36cc1d71af3f3ab69bf8fb3df2ad4a0c155ef/simmechanics/CMakeLists.txt#L165).

We should make sure that xsens chest IMU is also included in iCub >= 2.7, < 3 models.