ENH: add compareOp for three-way comparison

- similar to the \c <=> operator in C++20.
  Primarily for use when defining cascaded sort function objects.

src/OpenFOAM/primitives/Scalar/Scalar.H

@@ -405,6 +405,28 @@ inline Scalar stabilise(const Scalar s, const Scalar tol)
 
 // Specializations
 
+// Default definition in ops.H
+template<class T> struct compareOp;
+
+//- Compare scalar values
+template<>
+struct compareOp<Scalar>
+{
+    const Scalar tolerance;
+
+    //- Construct with specified tolerance (non-negative value)
+    compareOp(Scalar tol = ScalarVSMALL)
+    :
+        tolerance(tol)
+    {}
+
+    Scalar operator()(const Scalar& a, const Scalar& b) const
+    {
+        return (mag(a - b) <= tolerance) ? 0 : (a - b);
+    }
+};
+
+
 // Default definition in ops.H
 template<class T> struct equalOp;
 

src/OpenFOAM/primitives/ops/ops.H

@@ -216,6 +216,21 @@ WeightedOp(multiply, (weight*y))
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
+//- Three-way comparison operation of two parameters,
+//  similar to the \c <=> operator in C++20.
+//
+//  \return a negative value for less, a positive value for greater,
+//      and zero for equal value.
+template<class T>
+struct compareOp
+{
+    int operator()(const T& a, const T& b) const WARNRETURN
+    {
+        return (a < b) ? -1 : (b < a) ? 1 : 0;
+    }
+};
+
+
 //- General get operation to extract the 'name' from an object as a word.
 //  The default implementation uses the 'name()' method commonly used within
 //  OpenFOAM.