Get a list of nodes whose coordinates are closer than *radius* to *node*.

def geo_search(self, radius, node=None, center=False, **kwargs):
        '''Get a list of nodes whose coordinates are closer than *radius* to *node*.'''
        node = as_node(node if node is not None else self)

        G = self.subgraph(**kwargs)

        pos = nx.get_node_attributes(G, 'pos')
        if not pos:
            return []
        nodes, coords = list(zip(*pos.items()))
        kdtree = KDTree(coords)  # Cannot provide generator.
        indices = kdtree.query_ball_point(pos[node], radius)
        return [nodes[i] for i in indices if center or (nodes[i] != node)]