add a CSVConnector/CSVExpression classes to serialize to csv format

Current limitation: list/vectors are not serialized.

while here:
- move the output_type flag from the DbExpression to BaseExpression class
- add Model::to_text(...) methods with an Export parameter

src/flatconnector.cpp

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2018-2021, Tomasz Sowa
+ * Copyright (c) 2018-2023, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -67,8 +67,7 @@ FlatExpression * FlatConnector::get_expression()
 }
 
 
-
-void FlatConnector::to_text(pt::TextStream & stream, Model & model)
+void FlatConnector::to_text(pt::TextStream & stream, Model & model, Export exp)
 {
 	allocate_default_expression_if_needed();
 
@@ -82,8 +81,6 @@ void FlatConnector::to_text(pt::TextStream & stream, Model & model)
 }
 
 
-
-
 void FlatConnector::deallocate_expression()
 {
 	if( expression_allocated )