mouse selection of a set of points
matrix of size (2,n1). n1=n if the parameter n is given.
locate is used to get the coordinates of one or more
points selected with the mouse in a graphics window. The coordinates
are given using the current graphics scale.
If n>0, n points are selected and their coordinates are returned in
If n<=0, points are selected until the user clicks with the left button of the mouse which stands for stop. The last point (clicked with the left button) is not returned.
x=locate() is the same as
flag=1 a cross is drawn at the points where the mouse is clicked.