add "rewind()" member function to Text and Potential file reader classes

src/potential_file_reader.cpp

@@ -92,6 +92,13 @@ void PotentialFileReader::ignore_comments(bool value)
   reader->ignore_comments = value;
 }
 
+/** Reset file to the beginning */
+
+void PotentialFileReader::rewind()
+{
+  reader->rewind();
+}
+
 /** Read a line but ignore its content */
 
 void PotentialFileReader::skip_line()