ENH: make expressions::FieldAssociation a common enum

- use FACE_DATA (was SURFACE_DATA) for similarity with polySurface

ENH: add expression value enumerations and traits

- simple enumeration of standard types (bool, label, scalar, vector)
  that can be used as a value type-code for internal bookkeeping.

GIT: relocate pTraits into general traits/ directory

src/finiteVolume/expressions/patch/patchExprDriverI.H

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -36,10 +36,10 @@ inline Foam::label Foam::expressions::patchExpr::parseDriver::size
     {
         case FieldAssociation::POINT_DATA :
             return patch_.patch().nPoints();
-            break;
-        case FieldAssociation::SURFACE_DATA :
+
+        case FieldAssociation::FACE_DATA :
             return patch_.patch().size();
-            break;
+
         default:
             break;
     }