Data model interface:

	createAggregation(self, parent) Create child aggregation.
	createFieldCondition(self, parent) Create field condition.
	delete(self, item) Delete condition
	getAggregationType(self, item) Return aggregation type for item
	getChildren(self, item) Return aggregation children
	getField(self, item) Return field of item
	getFieldOperation(self, item) Return field operation
	getFieldsModel(self) Return fields model for data model
	getFieldValue(self, item) Return field value
	getParent(self, item) Return parent item for given
	getRoot(self) Return root condition
	invalidateFieldValue(self, item) Invalidate field value
	isAggregation(self, item) Return item is aggregation.
	setAggregationType(self, item, aggr_type) Set aggregation type for item Must generate ItemChanged event with field as argument
	setField(self, item, field) Set condition field.
	setFieldOperation(self, item, oper) Set field condition operation
	setFieldsModel(self, model) Called when user changed field model
	setFieldValue(self, item, value) Set field value for given item