ENH: add fieldLevel handling for surface writers (#2382)

- this can be used to apply a uniform field level to remove from
  a sampled field. For example,

      fieldLevel
      {
          "p.*"   1e5;        // Absolute -> gauge [Pa]
          T       273.15;     // [K] -> [C]
          U       #eval{ 10/sqrt(3) };  // Uniform mag(U)=10
      }

  After the fieldLevel has been removed, any fieldScale is applied.
  For example

      fieldScale
      {
          "p.*"   0.01;       // [Pa] -> [mbar]
      }

  The fieldLevel for vector and tensor fields may still need some
  further refinement.

tutorials/compressible/rhoSimpleFoam/squareBend/system/samplingDebug

@@ -10,7 +10,7 @@ debug
     writeControl  timeStep;
     writeInterval 1;
 
-    fields  (rho U);
+    fields  (p rho U);
 
     sampleScheme  cellPoint;
     interpolationScheme cellPoint;
@@ -24,6 +24,11 @@ debug
         {
             collateTimes  true;
             // collateTimes  false;
+            fieldLevel
+            {
+                "p.*"   1e5;
+                U       0;
+            }
         }
         raw
         {