fix bug in bellman-ford algo. remove DEBUG. rename `showneighbors`.

*   bellman ford was attempting to calc distance as:
    direct cost to neighbor + cost from **destination to neighbor**
    when it should have been calculating:
    direct cost to neighbor + cost from **neighbor to destination**
    subtle difference.
    bug appeared when testing not-fully-connected networks.

*   `showneighbors` cmd renamed to just `neighbors` for simplicity.

*   removes DEBUG as it is no longer of use.