Add train stop prioritization

Trains are currently incapable of adequately balancing products which need to be dumped. Prioritization is need so that dump able products do not back up, but are not dumped if consumers are available. I have only found two really bad workarounds. you can either build an additional station for each level of priority needed and a new dedicated line for that station, which also has to be built in parallel to the original station to avoid trains without destinations from blocking the lower priority trains. This takes up a lot of unnecessary space. When i ran into this issue this solution was completely unfeasible since i was not aware i needed this much space before i started building. The only other solutions are to either add every stop on the same line which causes trains to drive empty a lot. Or try to use train limits which causes jams unless each product has its own station. This means that trains are not really worth it currently, they either take too much space or are inefficient.