ENH: improve expression string expansions

- reuse more of stringOps expansions to reduce code and improve the
  syntax flexiblity.

  We can now embed "pre-calculated" values into an expression.
  For example,

       angle       35;
       valueExpr   "vector(${{cos(degToRad($angle))}}, 2, 3)";

  and the ${{..}} will be evaluated with the regular string evaluation
  and used to build the entire expression for boundary condition
  evaluation.

  Could also use for fairly wild indirect referencing:

       axis1   (1 0 0);
       axis2   (0 1 0);
       axis3   (0 0 1);
       index   100;
       expr   "$[(vector) axis${{ ($index % 3) +1 }}] / ${{max(1,$index)}}";

src/OpenFOAM/expressions/exprString/exprString.H

@@ -115,7 +115,25 @@ public:
     ~exprString() = default;
 
 
-    // Member Functions
+    // Static Member Functions
+
+        //- Inplace expansion with dictionary variables,
+        //- and strip C/C++ comments from the input
+        static void inplaceExpand
+        (
+            std::string& str,
+            const dictionary& dict,
+            const bool stripComments = true
+        );
+
+        //- Get and expand expression with dictionary entries,
+        //- optionally strip C/C++ comments from the input
+        static exprString getExpression
+        (
+            const word& name,
+            const dictionary& dict,
+            const bool stripComments = false
+        );
 
         //- Copy convert string to exprString.
         //  No expansions, know what you are doing.
@@ -125,6 +143,9 @@ public:
         //  No expansions, know what you are doing.
         inline static exprString toExpr(std::string&& str);
 
+
+    // Member Functions
+
         //- Inplace expansion with dictionary variables,
         //- and strip C/C++ comments from the input
         exprString& expand