Priority in train station groups

Right now, the train will go to the closest active station in a group.
It would be really useful if you could give the stations different priorities to make sure the important places always have the things they need.
It would also allow things like priority output of rock and slag to the places that need them (concrete slabs), and then having the rest be dumped with a low priority without needing to use two separate export stations for that.

Additionally, when you have set up a new mine, you might want to prioritize the old one, so that it get's emptied first, and the new one can take over seamlessly.

Similarly on output it would help to be able to prioritize suppliers. Excess water (wastewater treatment, exhaust scrubber) that needs to be used up (push into the system) versus shipped in water (that is meant to be used only if there is no other need) would fit this scenario. so both for input as output the priority (or push into system versus passively provide) would be useful

Additional suggestions for station group priority handling:
- "Dump" stop type – similar to the existing "Refueling" marker, trains would only visit a "Dump" stop if cargo needs to be disposed of and all regular unloading options on the line are full (e.g., rock/slag). This could help prevent clogging storage and production lines when surplus materials need to be removed.
- Balanced loading/unloading across the group – an option for trains to visit stations in a group in a way that distributes trips evenly among them, rather than purely based on distance.
- Visit-all behavior – an option for trains to visit as many stations in the group as possible until they are completely full (for pickup) or empty (for delivery), instead of heading to an unload station when only one trailer is full.
These features would work well in combination with the priority system mentioned above, allowing for much more flexible and efficient logistics setups.