線形値で境界を拡張する。dim次元のそれぞれの境界において配列を1格子だけ拡張し、
expand boundary with linear value. extend array with 1 grid at each boundary with dim th dimension, and assign th value which diffrential value between a grid short of boundary and boundary grid in original array. (on other wards, 2*z_{0}-z_{1} or 2*z_{n-1}-z_{n-2}: now _{i} represents the suffix of {i} th element in the dim th dimension of array. ).
引数
z (NArray): 境界を拡張したいNArray
dim (Numeric): 微分する次元を表すNumeric。逆にカウントされる数字(dim<0)も許されるが、 but z.rank ¡Üdim must be > 0.