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/Make/files

@@ -0,0 +1,3 @@
+Test-exprTraits.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-exprTraits

applications/test/exprTraits/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */