def distance(coordinates)
      coordinates = coordinates.center if coordinates.respond_to?(:center)
      Math.sqrt([center, coordinates].transpose.map {|a,b| (b - a)**2}.reduce {|d1,d2| d1 + d2})
    end
def distance(coordinates)
      coordinates = coordinates.center if coordinates.respond_to?(:center)
      Math.sqrt([center, coordinates].transpose.map {|a,b| (b - a)**2}.reduce {|d1,d2| d1 + d2})
    end
Compute euclidien distance.

 @param coordinates an array of coord or a Ball instance