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/utilities/mesh/conversion/ensightToFoam/ensightMeshReader.C

@@ -645,7 +645,7 @@ bool Foam::fileFormats::ensightMeshReader::readGeometry
 
 
     // Parse all
-    SubStrings<string> split;
+    SubStrings split;
 
     while (is.good())
     {