ENH: isLiteral() method for keyType and wordRe

- same as !isPattern(), but can be more readable.

- add wordRe enum state 'UNKNOWN', which has the identical value as
  'DETECT' but used for a return value.

src/functionObjects/field/zeroGradient/zeroGradient.C

@@ -133,9 +133,9 @@ bool Foam::functionObjects::zeroGradient::execute()
     // Check exact matches first
     for (const wordRe& select : selectFields_)
     {
-        if (!select.isPattern())
+        if (select.isLiteral())
         {
-            const word& fieldName = static_cast<const word&>(select);
+            const word& fieldName = select;
 
             if (!candidates.erase(fieldName))
             {