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/convertAreaFields.H

@@ -29,7 +29,7 @@ if (doFiniteArea)
     autoPtr<faMesh> faMeshPtr;
 
     const label nAreaFields =
-        objects.count(stringListOps::foundOp<word>(fieldTypes::area));
+        faObjects.count(stringListOps::foundOp<word>(fieldTypes::area));
 
     if (nAreaFields || withMeshIds)
     {
@@ -40,7 +40,7 @@ if (doFiniteArea)
     {
         const faMesh& areaMesh = faMeshPtr();
 
-        reportFields::area(Info, objects);
+        reportFields::area(Info, faObjects);
 
         const auto& pp = faMeshPtr->patch();
 
@@ -97,7 +97,7 @@ if (doFiniteArea)
         (
             writer,
             areaMesh,
-            objects,
+            faObjects,
             true // syncPar
         );