support stacked belts in the belt analyzer #314
Description
Followup for #268. Right now the belt analyzer will probably report nonsense values, e.g. "400% of the belt" etc. Stacked belts aren't accessible in 2.0 base however, so we may defer this until space age/the expansion.
Basically we have to decide how to expose it and modify belt-analyzer.lua to announce it. Everything else here probably already works and the information should already be available out of transport-belts.lua as well, but we may need to do more, e.g. so we can say "33.% of the belt, average stack size 2" or something along those lines.