Eigen using BLAS / LAPACK as backend

https://eigen.tuxfamily.org/dox/TopicUsingBlasLapack.html

After Eigen 3.3 version of BLAS and LAPACK can be called as a backend, it is also very simple to use:

Before referring Eigen library, first add the following macro definitions:

#define EIGEN_USE_BLAS

After at compile time, link the appropriate libraries can be,

g++ demo.cpp -lblas