ENH: add keyType::null and wordRe::null, add match method to string classes

- provides a more uniform interface to string list operations etc

STYLE: more complete docs for keyType

src/OpenFOAM/primitives/strings/string/string.H

@@ -82,6 +82,8 @@ public:
 
         static const char* const typeName;
         static int debug;
+
+        //- An empty string
         static const string null;
 
 
@@ -143,6 +145,9 @@ public:
         template<class String>
         static inline string quotemeta(const string&, const char quote='\\');
 
+        //- True when strings match literally
+        inline bool match(const std::string&) const;
+
         //- Avoid masking the normal std::string replace
         using std::string::replace;