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/dbexpression.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2018-2022, Tomasz Sowa
+ * Copyright (c) 2018-2023, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -53,11 +53,6 @@ public:
 
 	constexpr static const char * COLUMN_ROWS_COUNTER_POSTFIX = "_autoadded_rows_counter";
 
-
-	virtual void set_output_type(int output_type);
-	virtual int  get_output_type();
-
-
 	virtual void prepare_to_where_clause();
 
 	virtual DbExpression & group_or(pt::TextStream & stream);
@@ -86,7 +81,7 @@ public:
 
 protected:
 
-	int output_type;
+
 	std::vector<int> conjunctions;
 
 	bool can_field_be_generated(const FT & field_type);