Change liquid volume to 1, fixes waterskins

1 plastic bottle = 500ml = 2 volume = 2 units of water/drink, but liquids are defined with 2 volume in the jsons. This breaks waterskins, so I changed it to 1. Also tweaked large waterskin, it shouldn't take twice the volume of 2 normal waterskins while at the same time having 1 litre less capacity.