ENH: various enhancements for edge.

- support edge-ordering on construction, and additional methods:
  - sort(), sorted(), unitVec(), collapse()

- null constructor initializes with -1, for consistency with face,
  triFace and since it is generally much more useful that way.

- add some methods that allow edges to used somewhat more like hashes.
  - count(), found(), insert(), erase()

  Here is possible way to use that:

      edge someEdge;  // initializes with '-1' for both entries

      if (someEdge.insert(pt1))
      {
         // added a new point label
      }

      ... later

      // unmark point on edge
      someEdge.erase(pt2);

--

STYLE:

- use UList<point> instead of pointField for edge methods for flexibility.

  The pointField include is retained, however, since many other routines
  may be relying on it being included via edge.H