ENH: additional in-place clamp_min(), clamp_max() field methods

- these already existed for a single value, but now handle the full
  field. This is more memory-friendly.

      fld.clamp_min(lower);  OLD: fld = max(fld, lower);
      fld.clamp_max(upper);  OLD: fld = min(fld, upper);

applications/solvers/multiphase/VoF/setRDeltaT.H

@@ -53,6 +53,21 @@
         pimpleDict.getOrDefault<scalar>("maxDeltaT", GREAT)
     );
 
+
+    // The old reciprocal time scale field, with any damping factor
+    tmp<volScalarField> rDeltaT0_damped;
+
+    // Calculate damped value before applying any other changes
+    if
+    (
+        rDeltaTDampingCoeff < 1
+     && runTime.timeIndex() > runTime.startTimeIndex() + 1
+    )
+    {
+        rDeltaT0_damped = (scalar(1) - rDeltaTDampingCoeff)*(rDeltaT);
+    }
+
+
     volScalarField rDeltaT0("rDeltaT0", rDeltaT);
 
     // Set the reciprocal time-step from the local Courant number
@@ -114,20 +129,12 @@
         << gMin(1/rDeltaT.primitiveField())
         << ", " << gMax(1/rDeltaT.primitiveField()) << endl;
 
-    // Limit rate of change of time scale
+    // Limit rate of change of time scale (=> smallest reciprocal time)
     // - reduce as much as required
     // - only increase at a fraction of old time scale
-    if
-    (
-        rDeltaTDampingCoeff < 1.0
-     && runTime.timeIndex() > runTime.startTimeIndex() + 1
-    )
+    if (rDeltaT0_damped)
     {
-        rDeltaT = max
-        (
-            rDeltaT,
-            (scalar(1) - rDeltaTDampingCoeff)*rDeltaT0
-        );
+        rDeltaT.clamp_min(rDeltaT0_damped());
 
         Info<< "Damped flow time scale min/max = "
             << gMin(1/rDeltaT.primitiveField())