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   Finds the cost for going from startNode to nextNode. Assumes a 
   square grid.
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   Form:
   cost = GridPathCost( startLoc, goalLoc, d )
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   Inputs
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   startNode (1,1) Id of start location
   goalNode  (1,1) Id of next location
   d         (1,1) Data structure
                   .n      (1,1) Number of nodes

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   Outputs
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   cost      (1,:)  List of nodes in path 

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