add missing PartialReduxExpr::coeff(index) function
add missing PartialReduxExpr::coeff(index) function

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