ENH: use gMinMax() instead of separate gMin(), gMax()

- for reciprocal values, gMinMax() first and then calculate the
  reciprocal, which avoids creating temporaries

STYLE: prefer MinMax to separate min/max accounting

COMP: namespace qualify min/max for deltaT, CourantNo, etc (#3348)

applications/utilities/preProcessing/createBoxTurb/createBoxTurb.C

@@ -167,18 +167,24 @@ int main(int argc, char *argv[])
         Info<< "Generating kinetic energy field" << endl;
         volScalarField k("k", 0.5*magSqr(U));
         k.write();
+
+        auto limits = gMinMax(k);
+        auto avg = gAverage(k);
+
         Info<< "min/max/average k = "
-            << gMin(k) << ", " << gMax(k) << ", " << gAverage(k)
-            << endl;
+            << limits.min() << ", " << limits.max() << ", " << avg << endl;
     }
 
     {
         Info<< "Generating div(U) field" << endl;
         volScalarField divU(fvc::div(U));
         divU.write();
+
+        auto limits = gMinMax(divU);
+        auto avg = gAverage(divU);
+
         Info<< "min/max/average div(U) = "
-            << gMin(divU) << ", " << gMax(divU) << ", " << gAverage(divU)
-            << endl;
+            << limits.min() << ", " << limits.max() << ", " << avg << endl;
     }
 
     Info<< nl;

applications/utilities/preProcessing/setAlphaField/setAlphaField.C

@@ -223,11 +223,16 @@ int main(int argc, char *argv[])
     Info<< "Writing new alpha field " << alpha1.name() << endl;
     alpha1.write();
 
-    const scalarField& alpha = alpha1.internalField();
+    {
+        const auto& alpha = alpha1.primitiveField();
 
-    Info<< "sum(alpha*V):" << gSum(mesh.V()*alpha)
-        << ", 1-max(alpha1): " << 1 - gMax(alpha)
-        << " min(alpha1): " << gMin(alpha) << endl;
+        auto limits = gMinMax(alpha);
+        auto total = gWeightedSum(mesh.V(), alpha);
+
+        Info<< "sum(alpha*V):" << total
+            << ", 1-max(alpha1): " << 1 - limits.max()
+            << " min(alpha1): " << limits.min() << endl;
+    }
 
     Info<< "\nEnd\n" << endl;