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.