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.h

@@ -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
@@ -36,6 +36,7 @@
 #define headerfile_morm_src_flatconnector
 
 #include "textstream/textstream.h"
+#include "export.h"
 
 
 namespace morm
@@ -51,7 +52,7 @@ public:
 	FlatConnector();
 	virtual ~FlatConnector();
 
-	virtual void to_text(pt::TextStream & stream, Model & model);
+	virtual void to_text(pt::TextStream & stream, Model & model, Export exp);
 	virtual void set_expression(FlatExpression & expression);
 	virtual FlatExpression * get_expression();