ENH: separate registry and revised file locations for finite-area

- The internal storage location of finite-area changes from being
  piggybacked on the polyMesh registry to a having its own dedicated
  registry:

  * allows a clearer separation of field types without name clashes.
  * prerequisite for supporting multiple finite-area regions (future)

Old Locations:
```
   0/Us
   constant/faMesh
   system/faMeshDefinition
   system/faSchemes
   system/faSolution
```

New Locations:
```
   0/finite-area/Us
   constant/finite-area/faMesh
   system/finite-area/faMeshDefinition  (or system/faMeshDefinition)
   system/finite-area/faSchemes
   system/finite-area/faSolution
```

NOTES:
    The new locations represent a hard change (breaking change) that
    is normally to be avoided, but seamless compatibility handling
    within the code was found to be unworkable.

    The `foamUpgradeFiniteArea` script provides assistance with migration.

    As a convenience, the system/faMeshDefinition location continues
    to be supported (may be deprecated in the future).

applications/utilities/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2022 OpenCFD Ltd.
+    Copyright (C) 2016-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -781,20 +781,33 @@ int main(int argc, char *argv[])
             }
 
             IOobjectList objects;
+            IOobjectList faObjects;
 
             if (doConvertFields)
             {
-                // List of objects for this time
+                // List of volume mesh objects for this instance
                 objects =
                     IOobjectList(meshProxy.baseMesh(), runTime.timeName());
 
+                // List of area mesh objects (assuming single region)
+                faObjects =
+                    IOobjectList
+                    (
+                        runTime,
+                        runTime.timeName(),
+                        faMesh::dbDir(meshProxy.baseMesh(), word::null),
+                        IOobjectOption::NO_REGISTER
+                    );
+
                 if (fieldSelector && !fieldSelector().empty())
                 {
                     objects.filterObjects(fieldSelector());
+                    faObjects.filterObjects(fieldSelector());
                 }
 
                 // Remove "*_0" restart fields
                 objects.prune_0();
+                faObjects.prune_0();
 
                 if (!doPointValues)
                 {