線性算術的C++模板庫，Eigen 3.1 發布

Eigen 3.1 的改進記錄：

1. A new set of officially supported sparse modules for the representation, assembly, and solving of sparse problems, including many built-in and third-party sparse linear solvers.
   
2. The ability to seamlessly fallback to Intel MKL for some operations such as including matrix products, dense matrix decompositions, and math array operations.
   
3. These optional backends can be enabled at compile-time.
   
4. Some new coefficient and vector-wise operations, rank update/downdate for LLT and LDLT factorizations, closed form eigen-decompositions, and some memory and performance optimizations