Edge
==================================================

Definition::

    Node origin
    Node destination
    
    # There re various ways to traverse an edge. The options are listed below, to be used as content of `action_type`
    string ACTION_DRIVE=DRIVE
    string ACTION_OPEN_DOOR=OPEN_DOOR
    string ACTION_PUSH_OBJECT=PUSH_OBJECT
    string ACTION_USE_ELEVATOR=USE_ELEVATOR
    string action_type  # Any of ACTION_*
    
    float32 cost  # How expensive is is to travel along this edge?