Code review comment for lp:~widelands-dev/widelands/ai_productionsites_statistics

As I wrote on the forums, I think that this should be calculated when parsing the production programs. This way, we get more accurate statistics, and we can use it in the encyclopedia.

This has been in my head for a while and I plan to start working on this once the 2 prerequisite branches are in, so I'd rather not add new code that we'll then need to remove anyway.