add description of calculation of MAC costs in industry module
requested to merge github/fork/0UmfHxcvx5J7JoaOhFSs5mncnisTJJ6q/doc__MAC_cost_calculation into develop
Added description for the calculation of MAC costs, and a picture! Please check if that explanation makes sense. Only part of the text show up in the HTML, as goxygen is broken for me, so best check the source.