Put Ziprail Vehicle Unused Field Named Speed To Use

Makes unused ingame float field _speed for ZiprailVehicle class contibute to movementSpeed property

How movementSpeed is calculated without this mod: this._initializationParams.direction / (this.path.totalLength / this.parentController.speed

How movementSpeed is calculated with this mod: this._initializationParams.direction / (this.path.totalLength / (this.parentController.speed * this._speed)

Also hooks onto ctor of ZiprailVehicle to set _speed field to 1

Before doing changes checks if ZiprailVehicle class has field named "_speed" and return if detects none