Added several buildings related to gravship.
gravcore power generator
a power generator device modeled after the Gravcore Power Cell,it passively draws a small amount of energy from gravitational waves passing through the gravship.
gravship cabin door
a door that prevents depressurization in vacuum environments. It allows personnel to pass through while blocking gas leakage, and is considered a door by the system.
It separates rooms, can only be opened or closed manually, and slows down movement — how much it slows depends on the stuff of the door.
