ENH: use exprString expansions for #eval

- follows the principle of least surprise if the expansion behaviour
  for #eval and expressions (eg, exprFixedValue) are the same.  This
  is possible now that we harness the regular stringOps::expand()
  within exprString::expand()

applications/test/dictionary/testDictEval2

@@ -47,10 +47,10 @@ eval6b  #eval " pi() * 2 * ${{ ${unknown:-0} * ${{ 15 * 3 }} }} + 5";
 eval6c  #eval " pi() * 2 * ${{ -${unknown:-0} * ${{ 15 * 3 }} }} + 5";
 
 // Even this work
-eval7a   #eval " pi() * 1 * ${factor${{2*5}}:-100}";
+eval7a  #eval " pi() * 1 * ${factor${{2*5}}:-100}";
 
 // Even this work
-eval7b   #eval " pi() * 1 * ${factorXYZ${{2*5}}:-100}";
+eval7b  #eval " pi() * 1 * ${factorXYZ${{2*5}}:-100}";
 
 index   10;
 
@@ -58,7 +58,40 @@ eval8a  #eval " pi() * 2 * ${{ ${factor$index} + ${factor10} }}";
 
 eval8b  #eval " pi() * 2 * $factor * ${{ 100 }}";
 
-eval10a  #eval "vector(1,2,3) * 5";
+eval10a #eval "vector(1,2,3) * 5";
 
 
+// Slightly stranger ideas:
+
+axis1   (1 0 0);
+axis2   (0 1 0);
+axis3   (0 0 1);
+index   100;
+
+location #eval #{
+    400 *
+
+    // Numerator: use the index to get relevant suffix [1,2,3]
+    // to form the lookup of axis1,axis2,...
+    // Treat the lookup as a vector within the expression evaluation
+
+    $[(vector)   axis${{
+            ($index % 3) + 1 /* Evaluates: 1,2,3 from index */ }}
+     ]
+
+    // Denominator: divide by index with zero-division protection
+  / ${{max(1, $index)}}
+
+    // Same thing, but using expressions-syntax for variable lookup
+  / ${{max(1, $[index])}}
+#};
+
+
+// This is still a rather clunky syntax
+length #eval{ cbrt(mag($[(vector) location])) };
+
+
+// Remove evidence of some variables
+#remove ("axis[0-9]*" index)
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //