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/foamToEnsight/readFields.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -32,9 +32,10 @@ License
 
 Foam::label Foam::checkData
 (
-    const fvMesh& mesh,
+    const objectRegistry& obr,
     const instantList& timeDirs,
-    wordList& objectNames
+    wordList& objectNames,
+    const fileName& local
 )
 {
     // Assume prune_0() was used prior to calling this
@@ -43,6 +44,9 @@ Foam::label Foam::checkData
 
     for (const word& fieldName : objectNames)
     {
+        // // If prune_0() not previously used...
+        // if (objectNames.ends_with("_0")) continue;
+
         bool good = false;
 
         for (const instant& inst : timeDirs)
@@ -52,7 +56,8 @@ Foam::label Foam::checkData
                 (
                     fieldName,
                     inst.name(),
-                    mesh,
+                    local,
+                    obr,
                     IOobject::NO_READ,
                     IOobject::NO_WRITE,
                     IOobject::NO_REGISTER