ENH: additional methods and improvements to plane

- signedDistance() method is like distance() but retains
  the positive/negative sign for the side of the plane.

- the sign() method returns the sign as -1,0,+1 integer for
  classification purposes where it is important to distinguish between
  a zero value and a positive value (eg, for cutting). Optional
  tolerance can be supplied to round for zero.

- refactor and inlined simple and frequently used methods.

- add boundBox faceCentre() method, which can be useful for creating
  clipping planes from a bounding box.
  Relocated treeBoundBox faceNormals to boundBox since they apply
  equally there - the meaning of the faces (x-min, x-max, etc)
  is the same, even if the point addressing for the faces differs.

src/OpenFOAM/meshes/boundBox/boundBox.C

@@ -45,12 +45,22 @@ const Foam::boundBox Foam::boundBox::invertedBox
 const Foam::faceList Foam::boundBox::faces
 ({
     // Point and face order as per hex cellmodel
-    face{0, 4, 7, 3}, // x-min
-    face{1, 2, 6, 5}, // x-max
-    face{0, 1, 5, 4}, // y-min
-    face{3, 7, 6, 2}, // y-max
-    face{0, 3, 2, 1}, // z-min
-    face{4, 5, 6, 7}  // z-max
+    face({0, 4, 7, 3}),  // 0: x-min, left
+    face({1, 2, 6, 5}),  // 1: x-max, right
+    face({0, 1, 5, 4}),  // 2: y-min, bottom
+    face({3, 7, 6, 2}),  // 3: y-max, top
+    face({0, 3, 2, 1}),  // 4: z-min, back
+    face({4, 5, 6, 7})   // 5: z-max, front
+});
+
+const Foam::FixedList<Foam::vector, 6> Foam::boundBox::faceNormals
+({
+    vector(-1,  0,  0), // 0: x-min, left
+    vector( 1,  0,  0), // 1: x-max, right
+    vector( 0, -1,  0), // 2: y-min, bottom
+    vector( 0,  1,  0), // 3: y-max, top
+    vector( 0,  0, -1), // 4: z-min, back
+    vector( 0,  0,  1)  // 5: z-max, front
 });
 
 
@@ -107,8 +117,8 @@ Foam::boundBox::boundBox
 
 Foam::tmp<Foam::pointField> Foam::boundBox::points() const
 {
-    tmp<pointField> tpoints(new pointField(8));
-    pointField& pt = tpoints.ref();
+    auto tpt = tmp<pointField>::New(8);
+    auto& pt = tpt.ref();
 
     pt[0] = min_;                                   // min-x, min-y, min-z
     pt[1] = point(max_.x(), min_.y(), min_.z());    // max-x, min-y, min-z
@@ -119,7 +129,46 @@ Foam::tmp<Foam::pointField> Foam::boundBox::points() const
     pt[6] = max_;                                   // max-x, max-y, max-z
     pt[7] = point(min_.x(), max_.y(), max_.z());    // min-x, max-y, max-z
 
-    return tpoints;
+    return tpt;
+}
+
+
+Foam::tmp<Foam::pointField> Foam::boundBox::faceCentres() const
+{
+    auto tpts = tmp<pointField>::New(6);
+    auto& pts = tpts.ref();
+
+    forAll(pts, facei)
+    {
+        pts[facei] = faceCentre(facei);
+    }
+
+    return tpts;
+}
+
+
+Foam::point Foam::boundBox::faceCentre(const direction facei) const
+{
+    point pt = boundBox::midpoint();
+
+    if (facei > 5)
+    {
+        FatalErrorInFunction
+            << "face should be [0..5]"
+            << abort(FatalError);
+    }
+
+    switch (facei)
+    {
+        case 0: pt.x() = min().x(); break;  // 0: x-min, left
+        case 1: pt.x() = max().x(); break;  // 1: x-max, right
+        case 2: pt.y() = min().y(); break;  // 2: y-min, bottom
+        case 3: pt.y() = max().y(); break;  // 3: y-max, top
+        case 4: pt.z() = min().z(); break;  // 4: z-min, back
+        case 5: pt.z() = max().z(); break;  // 5: z-max, front
+    }
+
+    return pt;
 }
 
 
@@ -160,11 +209,11 @@ bool Foam::boundBox::intersects(const plane& pln) const
     bool below = false;
 
     tmp<pointField> tpts(points());
-    const pointField& pts = tpts();
+    const auto& pts = tpts();
 
     for (const point& p : pts)
     {
-        if (pln.sideOfPlane(p) == plane::NORMAL)
+        if (pln.sideOfPlane(p) == plane::FRONT)
         {
             above = true;
         }
@@ -208,9 +257,9 @@ bool Foam::boundBox::contains
         return true;
     }
 
-    forAll(indices, i)
+    for (const label pointi : indices)
     {
-        if (!contains(points[indices[i]]))
+        if (!contains(points[pointi]))
         {
             return false;
         }
@@ -250,9 +299,9 @@ bool Foam::boundBox::containsAny
         return true;
     }
 
-    forAll(indices, i)
+    for (const label pointi : indices)
     {
-        if (contains(points[indices[i]]))
+        if (contains(points[pointi]))
         {
             return true;
         }