GridTraverseCost:

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   Finds the cost of going from one node to another in a square grid.
   Diagonal moves are permitted.
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   Form:
   cost = GridTraverseCost( startNode, nextNode, d )
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   Inputs
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   startNode (1,1) Id of start location
   nextNode  (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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