Fix code in the of SixAxisFTOffsetEstimation in the examples folder of the matlab bindings

[fjandrad]

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I was running the examples that come for the matlab bindings just to check the installation I had was working properly and got the following result:

Running the example mentioned in the titles gives the following error:

Unrecognized method, property, or field 'toMatlab' for class 'SwigRef'.

Error in SwigRef/subsref (line 33)
        [varargout{1:nargout}] = builtin('subsref',self,s);

Error in SixAxisFTOffsetEstimation (line 148)
estContactForcesExtWrenchesEst.contactWrench(estimator.model().getLinkIndex('l_foot'),0).contactWrench().getLinearVec3().toMatlab()

[traversaro]

Probably something changed in all the removal of semantics classes in https://github.com/robotology/idyntree/pull/704 . In particular, now all the getLinearVec3 should all return the same GeomVector3 type, see https://github.com/robotology/idyntree/blob/17d5a7db7f7190944aa6d13f96daaf1d4615946c/src/core/include/iDynTree/Core/GeomVector3.h#L38 , so we should double check why that type don't get the toMatlab method added.