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This method actually opens a connection to the database.
	Any data specific to the connection (such as 
	a database handle created by the database API when opening the connection) must be stored in a private structure
	(which should be defined in the gda-<providername>.h file as a 
	<Providername>ConnectionData); this private structure's adress should be
	"attached" to the GdaConnection object using the
	gda_connection_internal_set_provider_data().
      
This method actually closes a connection previously opened by the open_connection() virtual method. This method is not mandatory for virtual providers.
The connection specific data for the database API can be retrieved using the gda_connection_internal_get_provider_data() method.