ENH: add ListPolicy uniformity enumeration

- defines values for EMPTY, UNIFORM, NONUNIFORM and MIXED
  that allow bitwise or reduction.

src/OpenFOAM/containers/Lists/policy/ListPolicy.H

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -88,6 +88,18 @@ template<> struct no_linebreak<word> : std::true_type {};
 template<> struct no_linebreak<wordRe> : std::true_type {};
 
 
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+//- Classification of list/container uniformity.
+//- Can use with bit-wise or reduction
+enum uniformity : unsigned char
+{
+    EMPTY = 0,          //!< An empty container
+    UNIFORM = 0x1,      //!< Container (non-empty) with identical values
+    NONUNIFORM = 0x2,   //!< Container (non-empty) with different values
+    MIXED = 0x3         //!< Mixed uniform/non-uniform (after reduction)
+};
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 } // End namespace ListPolicy