CONTOURDATA Extract Contour Data from Contour Matrix C.
CONTOUR, CONTOURF, CONTOUR3, and CONTOURC all produce a contour matrix
C that is traditionally used by CLABEL for creating contour labels.
S = CONTOURDATA(C) extracts the (x,y) data pairs describing each contour
line and other data from the contour matrix C. The vector array structure
S returned has the following fields:
S(k).level contains the contour level height of the k-th line.
S(k).numel contains the number of points describing the k-th line.
S(k).isopen is True if the k-th contour is open and False if it is closed.
S(k).xdata contains the x-axis data for the k-th line as a column vector.
S(k).ydata contains the y-axis data for the k-th line as a column vector.