Dynamic Autodiff should report the number of parameter blocks to the functor

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- In the dynamic autodiff'd cost functor, it would be nice to have the number 
of parameters in the operator() argument. This information is here duplicated, 
while it is known by Ceres when calling AddParameterBlock on the 
DynamicAutoDiffCostFunction object. Moreover, the current situation forces the 
creation of as many functor objets as there are variations on the number of 
parameters (or worse, using dummy parameters), while if the number of 
parameters was available then only one functor would be needed. Something like :
  template<typename T> bool operator() (const std::vector<const* T>* parameters, T* residuals) const;
or, for free-style arrays:
  template<typename T> bool operator() (const int num_parameters, T const* const* parameters, T* residuals) const;

Original issue reported on code.google.com by [email protected] on 28 Apr 2013 at 12:15