GI improvements on GdaDataModel, GdaDataModelIter, GdaHolder, GdaRow and GdaNumeric. Added API to GdaNumeric.

* Added API to GdaNumeric. Now must be considered as opaque
  struct
* Added GSEAL() macro as initial work to seal GdaNumeric
* Fixed some GObject Introspection annotations
* Fixed Vala bindings for GdaDataModel, GdaDataModelIter,
  GdaHolder, GdaRow and GdaNumeric