LINEINTRGAL Line Integral in a 2D Vector Field.
LINEINTRGAL(X,Y,U,V,C) computes the line integral along the lines given in cell array C.
X and Y define the coordinates of a rectangular grid over which
U and V are defined. X and Y must be monotonic and 2D plaid as
％ produced by MESHGRID. X, Y, U, and V must all be the same size.
假定已經有許多應用采用了程序1 - 1 5中所定義的C u r r e n c y類，現在我們想要對C u r r e n c y類
的描述進行修改，使其應用頻率最高的兩個函數A d d和I n c r e m e n t可以運行得更快，從而提高應
用程序的執行速度。由于用戶僅能通過p u b l i c部分所提供的接口與C u r r e n c y類進行交互，
％ A 2D homogeneous convection-diffusion case (u=exp(-ex*deta*x-ex*deta*y) with a square with
％ all Dirichlet boundary, note that reaction coefficient is not zero
％ by indirect BKM
