allow us to push dumping

right now if we mark somewhere as "dump" it only dumps if a truck has that item AND there are no storage places of higher interest.
however please let us set it so when we mark it as dump it will PULL from storage. right now to fill a hole with dirt from storage i have to disable all other active storing places but that one AND mark some storage as export.

obviously this would be on the mine control tower screen

Just place a control tower over it and make import from some storage. Usuall way is to have two directly connected storages nearby - one set to request that material, the other set to always send it away - but with export only to that control tower (or towers) - that way you will force material to be dumped and you can control priorities by setting priority on requester storage.