v0.1.0

Implements a new API for working with contour levels.

c = contours(xs, ys, zs) # a contour collection with levels selected automatically
for lvl in levels(c)
    l = level(lvl) # the z-value of the current contour level
    for line in lines(lvl) # each contour level might consist of multiple, disjoint paths
       x, y = coordinates(line) # coordinates for the line segment of the current path
       # hypothetical example: plot(x, y; color = l)
    end
end