STYLE: use string substr instead of string::operator()

- makes the purpose clearer.
  In some places, string::resize() is even simpler.

- use C++11 string::back() in preference to str[str.size()-1]

src/sampling/sampledSurface/readers/ensight/ensightSurfaceReader.C

@@ -186,7 +186,7 @@ void Foam::ensightSurfaceReader::readCase(IFstream& is)
         //   surfaceName.****.fieldName
         // This is not parser friendly - simply take remainder of buffer
         label iPos = iss.stdStream().tellg();
-        fieldFileName = buffer(iPos, buffer.size() - iPos);
+        fieldFileName = buffer.substr(iPos);
         size_t p0  = fieldFileName.find_first_not_of(' ');
         if (p0 == string::npos)
         {