Find a clean way to generate a simmechanics-generated model with slightly different kinematics

[traversaro]

For example, it will be useful to generate a Simmechanics-based model with a kinematics of the arm modified to have a v1 arm (see https://github.com/robotology-playground/icub-model-generator/issues/50) or a v2 head with a v2.5 IMU (see https://github.com/robotology-playground/icub-model-generator/issues/2). For this kind of modifications, I think it is relatively reasonably to assume that the inertial parameters do not change.