access trainstop for waiting train

[Choumiko]

Having something like LuaTrain.station which contains the trainstop a train is currently waiting at would be really helpfull.
Right now it can get really messy if the train has frontmovers and backmovers and is toggled between auto- and manualmode (for whatver reason) while waiting at a station.
LuaTrain should be nil if the train is not at a station/stop (trainstop or station? )

[Rseding91]

Added for 0.13.14.