ENH: simplify SubStrings class/handling (remove template parameters)

- previous code used derived string iterators, but these would
  be largely ignored anyhow since the underlying std::sub_match
  str() method would just yields a std::string anyhow.

  The SubStrings::str(size_t) method wasn't used in any code, so now
  just use std::string iterators only.

  This change simplfies overall handling, since it removes an unneeded
  template dependency.

applications/test/stringSplit/Test-stringSplit.cxx

@@ -43,7 +43,7 @@ template<class StringType>
 void printSubStrings
 (
     const StringType& str,
-    const SubStrings<StringType>& split
+    const SubStrings& split
 )
 {
     Info<< "string {" << str.size() << " chars} = " << str << nl