In the original version of this article, the figure captions were captured in the wrong order.
The caption printed under Figure 2 is the caption that goes with fig. 4 on p. 334.
The caption printed under Figure 3 is the caption that goes with fig. 2 on p. 333.
The caption printed under Figure 4 is the caption that goes with fig. 3 on p. 333.
The authors apologise for this error.