dynamicsRegressorsGenerator cleanup

[traversaro]

We have to prepare a cleanup of the dynamicsRegressorsGenerator cleanup, to enable wrapping it with SWIG.

Key points:

• [x] Eliminate KDL dependency (initially just from headers, pimpling the actual implementation)
• [x] Load regressor structure and parameters from file, instead of hardcoding configuration at the C++ level. This seem really a JSON job, but for the time being I would avoid adding dependencies and I would go for a basic xml-based format.
• [x] Use the sensors classes to set/get sensors readings.
• [x] Use VectorDynSize and MatrixDynSize to wrap Eigen data structures to enable the use of SWIG.

For the future:

• [ ] Encode the difference between links and frames at the iDynTree data structure level, so we can avoid hardcoding in the configuration the "fake" links.

[traversaro]

Key points all done in http://wiki.icub.org/codyco/dox/html/classiDynTree_1_1Regressors_1_1DynamicsRegressorGenerator.html . For the future point still a todo, but not directly related to this issue.