add description of calculation of MAC costs in industry module

Michaja Pehlrequested 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.

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