Character or function to declare distance method.
If method is character, method must be "mahalanobis" or
"euclidean", "maximum", "manhattan", "canberra", "binary"
or "minkowisk".
If method is one of "euclidean", "maximum",
"manhattan", "canberra", "binary" or "minkowisk", see
dist for details,
because this function as used to compute the distance.
If method="mahalanobis", the mahalanobis distance
is computed between neighbour areas.
If method is a function, this function is
used to compute the distance.