make StdVector be a wrapper around it if EIGEN_USE_NEW_STDVECTOR is defined
otherwise StdVector doesn't change ---> compatibility is preserved
backport unit-test
*Add Eigen/StdVector header.
Including it #includes<vector> and "Core" and generates a partial
specialization of std::vector<T> for T=Eigen::Matrix<...>
that will work even with vectorizable fixed-size Eigen types
(working around a design issue in the c++ STL)
*Add unit-test
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