function [alpha]=angle2d(data)
Description: Calculates the angle between 2 vectors (given by pairs of points)
in 2 dimensions.
Input: data: data = [P1x P1y P2x P2y P3x P3y P4x P4y]
Note that "data" can have several rows (e.g. different time
points).
Output: alpha: angle (in deg) between the vectors P1-P2 and P3-P4
Author: Christoph Reinschmidt, HPL, The University of Calgary
Date: October, 1994
Last Changes: November 28, 1996
Version: 1.0