The authors regret an error in the text in the article.
On page 15, “element B” should read “element A”, and “element E” should read “element C” in the following sentence:
“Those correspond to the flux that enters the material from element B in figure 9 and the outward flux exiting from the material, i.e. the outward element E in the same figure.”
This has been corrected to:
“Those correspond to the flux that enters the material from element A in figure 9 and the outward flux exiting from the material, i.e. the outward element C in the same figure.”
The authors apologise for the error.