ENH: add exprField function object

- provides a simple means of defining/modifying fields. For example,

  ```
  <name1>
  {
      type    exprField;
      libs    (fieldFunctionObjects);
      field   pTotal;

      expression  "p + 0.5*(rho*magSqr(U))";
      dimensions  [ Pa ];
  }
  ```
  It is is also possible to modify an existing field.
  For example, to modify the previous one.
  ```
  <name2>
  {
      type    exprField;
      libs    (fieldFunctionObjects);
      field   pTotal;
      action  modify;

      // Static pressure only in these regions
      fieldMask
      #{
          (mag(pos()) < 0.05) && (pos().y() > 0)
       || cellZone(inlet)
      #};
      expression  "p";
  }
  ```

  To use as a simple post-process calculator, simply avoid storing the
  result and only generate on write:
  ```
  <name2>
  {
      store            false;
      executionControl none;
      writeControl     writeTime;
      ...
  }
  ```

tutorials/compressible/rhoSimpleFoam/squareBend/system/derivedFields

@@ -24,4 +24,42 @@ derivedFields
 }
 
 
+exprField1
+{
+    // Mandatory entries
+    type     exprField;
+    libs     (fieldFunctionObjects);
+    field    pTot;
+    /// action new;
+    ///autowrite       false;
+    ///store           true;
+    useNamePrefix   false;
+
+    // Use expression to define total pressure
+
+    executeControl  timeStep;
+    writeControl    none;
+
+    expression  "p + 0.5*(rho * magSqr(U))";
+    dimensions  [ Pa ];
+}
+
+
+exprField1.mod0
+{
+    // Mandatory entries
+    type    exprField;
+    libs    (fieldFunctionObjects);
+    field   pTot;
+    action  modify;
+
+    executeControl  timeStep;
+    writeControl    writeTime;
+
+    // Static pressure only in these regions
+    fieldMask   "(mag(pos()) < 0.05) && (pos().y() > 0) || cellZone(inlet)";
+    expression  "p";
+}
+
+
 // ************************************************************************* //

tutorials/compressible/rhoSimpleFoam/squareBend/system/sampling

@@ -69,7 +69,7 @@ sampled
 
     surfaceFormat   none;
 
-    fields      (p rho U T rhoU pTotal);
+    fields      (p rho U T rhoU pTotal pTot);
 
     // Remove derived fields we created prior
     removeFieldsOnExecute   (rhoU pTotal);
@@ -182,7 +182,7 @@ areaAverage
     operation       weightedAreaAverage;
     weightField     rhoU;
     weightFields    ( rho U none );  // 2012 and later
-    fields          ( p pTotal );
+    fields          ( p pTotal pTot );
 }
 
 areaIntegrate