ENH: overhaul ensight handling (#1579)

- includes restructuring and simplification of low-level ensight part
  handling and refactor of backends to improve code reuse.

foamToEnsight
-------------

  * new cellZone support.
    This was previously only possible via a separate foamToEnsightParts
    utility that was not parallelized.

  * support for point fields.

  * `-nearCellValue` option (as per foamToVTK)

  * data indexing now uses values from the time index.
    This is consistent with the ensightWrite function object and
    can help with restarts.

  * existing ensight directories are removed, unless the -no-overwrite
    option is supplied

foamToEnsightParts
------------------
  * now redundant and removed.

ensightOutputSurface (new class)
--------------------------------
  * a lightweight wrapper for point/face references that is tailored
    for the ensightSurfaceWriter. It uses compact face/point information
    and is serial only, since this is the format requirements from the
    surfaceWriter class.

ensightMesh (revised class)
---------------------------
  * now only holds a polyMesh reference, which removes its dependency
    on finiteVolume and allows it to be relocated under fileFormats
    instead of conversion.

Removed classes: ensightParts, ensighPartFaces, ensightPartCells

- these were used by foamToEnsightParts, but not needed anymore.

applications/utilities/postProcessing/dataConversion/foamToEnsight/getTimeIndex.H

@@ -0,0 +1,40 @@
+// Read time index from */uniform/time, but treat 0 and constant specially
+// or simply increment from the '-index' option if it was supplied
+
+label timeIndex = 0;
+{
+    if (doConsecutive)
+    {
+        timeIndex = indexingNumber++;
+    }
+    else if
+    (
+        runTime.timeName() != runTime.constant()
+     && runTime.timeName() != "0"
+    )
+    {
+        IOobject io
+        (
+            "time",
+            runTime.timeName(),
+            "uniform",
+            runTime,
+            IOobject::READ_IF_PRESENT,
+            IOobject::NO_WRITE,
+            false  // no register
+        );
+
+        if (io.typeHeaderOk<IOdictionary>(true, false))
+        {
+            io.readOpt() = IOobject::MUST_READ_IF_MODIFIED;
+            IOdictionary timeObject(io);
+
+            timeObject.readEntry("index", timeIndex);
+        }
+        else
+        {
+            Info<< "skip ... missing entry " << io.objectPath() << endl;
+            continue;
+        }
+    }
+}