Implement forwardDynamics

I initially wanted to implement this, but it turned out at this moment it is not strictly required.

So I will dump the only code I wrote in here:
~~~
/**
* @brief Compute the free floating forward dynamics.
*
* This method computes \f$ M(q)^{-1} (\tau_{gen} - C(q, \nu) \nu - G(q)) \f$.
*
* The semantics of the generalized torques and depend on the choosen FrameVelocityRepresentation .
*
* The state is the one given set by the setRobotState method.
*
* In particular $\tau_{gen}$ is composed by stacking the inputs totalBase6DForce and totalJointTorques.
*
* \warning This method is not implemented in iDynTree::KinDynComputatons, it is just implemented in
* iDynFor as it is easy to obtain it from the underlying pinocchio implementation.
*
* @param[in] totalBase6DForce the total base 6D force due to external 6D forces on all links
* @param[out] totalJointTorques the total joint torque, due to internal torques and torques induced by the external 6D forces on all links
* @return true if all went well, false otherwise
*/
bool forwardDynamics(const Eigen::Ref& totalBase6DForce,
const Eigen::Ref& totalJointTorques);

~~~

Start by reviewing the proposed forwardDynamics signature and the existing iDynFor implementation that the issue references, then compare it with iDynTree::KinDynComputations and the underlying pinocchio behavior. The issue names no files or tests, so identify the relevant dynamics entry points and establish tests for the documented free-floating dynamics semantics before implementation.

Written by the indexing model from the issue text.