add specialization of ei_ref_selector for Array (fix a big perf issue \!)
add specialization of ei_ref_selector for Array (fix a big perf issue \!)

rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen,madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen