Eigen: Wrapper for SIMD instructions

From @Rikrd at http://blog.wolfire.com/2010/09/SIMD-optimization:

If you make extensive use of linear algebra (matrices, vectors, etc.) you may consider using Eigen. It is a header only library and it implements quite a few linear algebra operations in a very intuitive API and it uses whatever backend is available (simple loop unrolling, vectorizing with SSE/SSE2/SSE3..., Altivec, etc...). It may save you a lot of vectorization work... and it is just really fun to write stuff using Eigen.