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

applications/test/edges/Make/files

@@ -0,0 +1,3 @@
+Test-edges.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-edges

applications/test/edges/Test-edges.C

@@ -0,0 +1,114 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2017 OpenCFD Ltd.
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-edges
+
+Description
+    Simple tests for edges
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "edgeList.H"
+
+using namespace Foam;
+
+void printInfo(const edge& e)
+{
+    Info<< "edge: " << e << " count:" << e.count() << nl;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+//  Main program:
+
+int main(int argc, char *argv[])
+{
+    edge e1;
+    printInfo(e1);
+    Info<<"has '2'? " << e1.found(2) << endl;
+
+    edge e2(1, 2);
+    printInfo(e2);
+    Info<<"has '2'? " << e2.found(2) << endl;
+
+    edge e3{2, 3};
+    printInfo(e3);
+    Info<<"has '2'? " << e3.found(2) << endl;
+
+    edge e4(4, 4);
+    printInfo(e4);
+    Info<<"has '2'? " << e4.found(2) << endl;
+
+    Info<<"collapse? -> " << e4.collapse() << endl;
+    printInfo(e4);
+
+    Info<< e3 << " connects " << e2 << " => " << e2.connects(e3) << endl;
+
+    Info<< nl << "hash-like functionality" << nl;
+
+    // doesn't work e4 = -1;
+    e4.start() = e4.end() = -1;
+
+    printInfo(e4);
+    for (label i : {2, -1, 2, 1, 4, 1, 2, 3})
+    {
+        bool ok = e4.insert(i);
+        Info<< "insert(" << i << ") = " << ok << " resulting ";
+        printInfo(e4);
+    }
+
+    for (label i : {-1, 0, 1, 3})
+    {
+        bool ok = e4.erase(i);
+        Info<< "erase(" << i << ") = " << ok << " resulting ";
+        printInfo(e4);
+    }
+
+    for (label i : {-1, 0, 1, 3})
+    {
+        bool ok = e4.insert(i);
+        Info<< "insert(" << i << ") = " << ok << " resulting ";
+        printInfo(e4);
+    }
+    e4.flip();
+    Info<< "flipped ";
+    printInfo(e4);
+
+    for (label i : {-1, 0, 1, 3})
+    {
+        bool ok = e4.erase(i);
+        Info<< "erase(" << i << ") = " << ok << " resulting ";
+        printInfo(e4);
+    }
+
+    e4.sort();
+    Info<< "sorted ";
+    printInfo(e4);
+
+    return 0;
+}
+
+
+// ************************************************************************* //