Simple QOL improvement for Loaders

[AndreyKl]

The process of loading rockets has some issues:

1. It's near impossible to know what already is in a rocket using automation. Not a problem early on but as rockets become more complicated, as player starts mixing gasses or other cargo types in a single rocket and sets up more automation this issue raises its head. Valve meters help a lot, but if rocket still has some oxygen in inventory, loading 600kg of oxygen again will go over desired limit. It's possible to unload a rocket first, then load it again, but it's mostly impractical due to tiny reservoir storage. It's possible to cap cargo at 600kg, but often said gas container also doubles as co2 storage for emptying cabin or steam storage to simplify refueling at a destination. Or the opposite issue: some oxygen might get drained into cabin and cargo will end up under desired capacity. Same can be seen when loading your rocket for colonization: you are adding various materials into your cargo, some minerals for building, some plastic, some medicines, metals. Yet if something wasn't used from previous trips (ex: destination had plenty of needed metals), we are back to needing to unload rocket before loading it again.

2. Another issue to consider: on planetoid A there are no easy oxygen sources, you only want to make sure rocket has enough oxygen for the trip. Planetoid B has plenty of oxygen production and you want to top off your rocket's oxygen storage. But unless you manually change rocket's capacity on each planetoid, it's Very problematic to achieve via automation.

3. And finally rocket platforms that share multiple loaders. If you have two steam rockets and want both to have 200kg of steam in cargo, with the rest being oxygen, loaders won't be of much help, you are better off loading steam through a cabin+gas intake to make sure it gets to specific rocket instead of using loaders. 

Suggestion:

1. Add a limit to cargo loaders.

Ideally this should be a limit per material per rocket (so that if cargo has 1500kg of oxygen and 200kg steam, loader loads 100 kg of oxygen despite cargo module having over 1600kg total).

2. Add automation output port to loaders that sends a signal when cargo is filled to desired capacity.

Ideally 'Signal' should check limit based on material that is currently in loader's inventory, loader only sends signal if it has something in inventory. If two rockets are docket, it sends signal only when both reached the limit. This will make it possible to know how filled the rocket is.

3. Drop down to select specific platform if multiple platforms are attached.

Probably not as important as the rest, but still would be nice to have.

P.S. This won't help with the dockmaster's nightmare that thinks it's an orbital drop-off module. But I think it will help with automating loading a lot.