Cumulative Cubic Integration

Similar to cumtrapz but third order accuracy. Unequal x spacing allowed.

z = cumint3(x,y) approximates the integrals from x1 to x2 of a cubic polynomial through points (x0,y0), (x1,y1), (x2,y2), (x3,y3) for sliding four-point sets, and produces z as the cumulative values of these in a manner similar to cumtrapz. End intervals use cubics with two other points to the same side. Unequal intervals in x values are allowed. x, y, and z are column vectors of the same length which can be any amount not less than four.