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

applications/test/exprTraits/Test-exprTraits.C

@@ -0,0 +1,79 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2021 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
+
+Description
+    Basic tests of expression traits
+
+\*---------------------------------------------------------------------------*/
+
+#include "IOstreams.H"
+#include "ITstream.H"
+#include "exprTraits.H"
+#include "uLabel.H"
+#include "error.H"
+#include "stringList.H"
+
+using namespace Foam;
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+template<class T>
+void printTraits()
+{
+    const auto typeCode = exprTypeTraits<T>::value;
+
+    Info<< "type " << pTraits<T>::typeName
+        << " code:" << int(typeCode)
+        << " name:" << exprTypeTraits<T>::name;
+
+    if (pTraits<T>::typeName != word(exprTypeTraits<T>::name))
+    {
+        Info<< " (UNSUPPORTED)";
+    }
+
+    Info << endl;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main()
+{
+    Info<< nl << "Traits:" << nl;
+
+    printTraits<word>();
+    printTraits<string>();
+    printTraits<bool>();
+    printTraits<label>();
+    printTraits<scalar>();
+    printTraits<vector>();
+    printTraits<tensor>();
+    printTraits<symmTensor>();
+    printTraits<sphericalTensor>();
+
+    const auto getName = nameOp<expressions::valueTypeCode>();
+
+    Info<< nl;
+
+    Info<< "Name of typeCode: "
+        << Foam::name(expressions::valueTypeCode::type_scalar) << nl;
+
+    Info<< "Name of typeCode: "
+        << getName(expressions::valueTypeCode::type_bool) << nl;
+
+
+    Info<< nl << "Done" << nl;
+    return 0;
+}
+
+
+// ************************************************************************* //