move the inertias from node classes to robot class

I want to be able to change a body inertia at runtime (during an init
stage).

Before this commit, inertias were stored within the Node classes

    class Node0 {
    public:
      Node0();
      static const int id = 0;
      static const std::string joint_name;
      static const std::string body_name;
      static const Spatial::Transform Xt;
      static const int q_idx = 0;
      typedef RevoluteAxisAnyJoint Joint;
      static const int parent_id = -1;
      static const int child0_id = 1;
      static const int child1_id = -2;
      static const int child2_id = -2;
      static const int child3_id = -2;
      static const int child4_id = -2;
      static const Spatial::Inertia I; // in body frame
      Spatial::TransformT<Spatial::rm_mul_op<Spatial::RotationMatrix, Spatial::RotationMatrix>::rm> sXp;
      Eigen::Matrix<FloatType, 6, Joint::NBDOF> joint_F; // used by crba
      Joint joint;
      Body body;
    };

and were static const. I could have just removed the const to get the
ability to update it at runtime. The problem is that doing so does not
enable me to write a simple setter like this:

    template <Robot>
    setInertia(Robot &robot, int node_id, const Spatial::Inertia &inertia)
    {
      typedef typename Nodes<Robot, node_id>::type Node;
      node::I = inertia;
    }

because the node_id, as a template parameter, should be known at
compile time. A solution would be to do

    setInertias(Robot &robot, std::vector<Spatial::Inertia> &inertia)
    {
      // use boost::fusion::for_each to set all the inertias
    }

or to use boost::fusion::for_each to iterate (at compile time) over all
the nodes and on set (doing the test at run time) the one with the
matching node_id.

Instead I chose to store all the inertias in an simple array instead of a
fusion vector.

Benchmarks show it has no impact on performance.

We could use the same method to ease the retrieval of other values at
run time: the body poses, velocities, etc. as long as all the elements
have the same type, and are never used as template parameters.