Method forward

tests/problem.cpp:34–37

      : ExplicitDynamicsModelTpl<double>(space, space->ndx()) {}

  void forward(const ConstVectorRef &x, const ConstVectorRef &u,
               ExplicitData &data) const {
    space_next().integrate(x, u, data.xnext_);
  }

  void dForward(const ConstVectorRef &x, const ConstVectorRef &u,
                ExplicitData &data) const {