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;
      ...
  }
  ```