Cleanup distributed inertial sensors implementations

[traversaro]

In #304 , @nunoguedelha contributed a plugin to simulate distributed inertials as the one that can be found on the iCub robot that mount an ETH electronics with MTB board whose version is newer than or equal to 3.

Unfortunately the interfaces of such sensors is currently quite iCub-specific, and the plugin for now is iCub-specific as well. Once the new appropriate interfaces will be developed (see robotology/yarp#1273) and once the iCub distributed inertial sensors will be migrated to them, we should "generalize" the distributed inertial plugin as well. Open this issue as a reference.