ENH: cleanup autoPtr class (issue #639)

Improve alignment of its behaviour with std::unique_ptr

  - element_type typedef
  - release() method - identical to ptr() method
  - get() method to get the pointer without checking and without releasing it.
  - operator*() for dereferencing

Method name changes

  - renamed rawPtr() to get()
  - renamed rawRef() to ref(), removed unused const version.

Removed methods/operators

  - assignment from a raw pointer was deleted (was rarely used).
    Can be convenient, but uncontrolled and potentially unsafe.
    Do allow assignment from a literal nullptr though, since this
    can never leak (and also corresponds to the unique_ptr API).

Additional methods

  - clone() method: forwards to the clone() method of the underlying
    data object with argument forwarding.

  - reset(autoPtr&&) as an alternative to operator=(autoPtr&&)

STYLE: avoid implicit conversion from autoPtr to object type in many places

- existing implementation has the following:

     operator const T&() const { return operator*(); }

  which means that the following code works:

       autoPtr<mapPolyMesh> map = ...;
       updateMesh(*map);    // OK: explicit dereferencing
       updateMesh(map());   // OK: explicit dereferencing
       updateMesh(map);     // OK: implicit dereferencing

  for clarity it may preferable to avoid the implicit dereferencing

- prefer operator* to operator() when deferenced a return value
  so it is clearer that a pointer is involve and not a function call
  etc    Eg,   return *meshPtr_;  vs.  return meshPtr_();

applications/utilities/parallelProcessing/reconstructParMesh/reconstructParMesh.C

@@ -257,7 +257,7 @@ autoPtr<mapPolyMesh> mergeSharedPoints
 
     if (returnReduce(pointToMaster.size(), sumOp<label>()) == 0)
     {
-        return autoPtr<mapPolyMesh>(nullptr);
+        return autoPtr<mapPolyMesh>();
     }
 
     polyTopoChange meshMod(mesh);
@@ -268,7 +268,7 @@ autoPtr<mapPolyMesh> mergeSharedPoints
     autoPtr<mapPolyMesh> map = meshMod.changeMesh(mesh, false, true);
 
     // Update fields. No inflation, parallel sync.
-    mesh.updateMesh(map);
+    mesh.updateMesh(map());
 
     // pointProcAddressing give indices into the master mesh so adapt them
     // for changed point numbering.
@@ -699,7 +699,7 @@ int main(int argc, char *argv[])
                 (
                     masterMesh[proci],
                     meshToAdd,
-                    couples
+                    couples()
                 );
 
                 // Added processor
@@ -738,7 +738,7 @@ int main(int argc, char *argv[])
                     (
                         masterMesh[proci],
                         masterMesh[next],
-                        couples
+                        couples()
                     );
 
                     // Processors that were already in masterMesh