ENH: improve surfaceFieldValue sampling and writing (#1999)

- ensure surface writing is time-step and nFields aware.
  This avoids overwriting (ignoring) previous output fields.

- allow sampled surfaces to be used for weight fields as well.
  Not sure why this restriction was still there.

- remove old compatibility reading of orientedFields.
  Last used in v1612, now removed.

- only use face sampling. For surfaceFieldValue we can only do
  something meaningful with face values.

ENH: modify interface methods for surfaceWriter

- replace direct modification of values with setter methods.
  Eg,
     old: writer.isPointData() = true;
     new: writer.isPointData(true);

  This makes it possible to add internal hooks to catch state changes.

ENH: allow post-construction change to sampledSurface interpolation

- rename interpolate() method to isPointData() for consistency with
  other classes and to indicate that it is a query.

- additional isPointData(bool) setter method to change the expected
  representation type after construction

- remove 'interpolate' restriction on isoSurfacePoint which was
  previously flagged as an error but within sampledSurfaces can use
  sampleScheme cellPoint and obtain representative samples.
  Relax this restriction since this particular iso-surface algorithm
  is slated for removal in the foreseeable future.

src/functionObjects/utilities/areaWrite/areaWrite.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019-2020 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -306,7 +306,7 @@ bool Foam::areaWrite::write()
 
 
         // Propagate field counts (per surface)
-        outWriter.nFields() = nAreaFields;
+        outWriter.nFields(nAreaFields);
 
 
         // Begin writing
@@ -351,7 +351,7 @@ void Foam::areaWrite::expire()
     {
         surfaceWriter& writer = *(iter.val());
         writer.expire();
-        writer.mergeDim() = mergeDim;
+        writer.mergeDim(mergeDim);
     }
 }