Grab all distances from nodes to POIs

When collecting all the distances from nodes to POIs the accessibility code was stopping after collecting the requested number of POIs. That assumes that the distances are being collected in sorted order and that some other shorter distance isn't later in the list. That assumption turned out to be not true, so the code needs to collect all the distances before sorting them. See particularly this diff: https://github.com/synthicity/pandana/commit/32883591a5edf803be2ddedf659bc71fb7c79f2a?w=1.