gradual loading doesn't work as expected with reservation

[fonsinchen]

Cargo is always reserved so that each vehicle is filled up in the order they're processed. So the first vehicles in a consist are reserved up to their capacity while the last ones may be left empty. If cargo is loaded from reservations then there is no "gradual loading". This might be fixed by restricting the amount of cargo reserved in ReserveConsist in economy.cpp.

[fonsinchen]

I won't fix this one as the difference in loading speed will only be noticable in the most extreme corner cases where new cargo appears just at the rate the consist is able to load with all vehicles in parallel. If cargo appears at a slower rate the consist will be able to load it without using all vehicles in parallel. If it appears at a faster rate the cargo will pile up and in the next round all vehicles will reserve cargo which leads to parallel loading.

One can argue it's even better to leave vehicles empty and load others fully in order to have free wagons for refitting at a later station.