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/baseexpression.cpp

@@ -51,6 +51,7 @@ BaseExpression::BaseExpression()
 	clear();
 }
 
+
 BaseExpression::~BaseExpression()
 {
 }
@@ -61,6 +62,7 @@ void BaseExpression::clear()
 	out_stream = nullptr;
 	is_first_field = false;
 	work_mode = 0;
+	output_type = 0;
 	use_prefix = false;
 }
 
@@ -77,6 +79,18 @@ int BaseExpression::get_work_mode()
 }
 
 
+void BaseExpression::set_output_type(int output_type)
+{
+	this->output_type = output_type;
+}
+
+
+int BaseExpression::get_output_type()
+{
+	return output_type;
+}
+
+
 
 pt::TextStream * BaseExpression::get_text_stream()
 {
@@ -184,6 +198,13 @@ bool BaseExpression::can_field_model_be_generated(bool has_model_primary_key, co
 }
 
 
+bool BaseExpression::can_field_list_be_generated(const FT &)
+{
+	return true;
+}
+
+
+
 bool BaseExpression::should_field_model_be_generated_as_null(bool has_model_primary_key, const FT & field_type)
 {
 	return false;