DOC: Data model tutorial.

src/doc/tutorials/data_model_tutorial.rst

src/doc/tutorials/img/data_model_and_data.png

src/doc/tutorials/img/data_model_and_data.svg

src/doc/tutorials/img/edit_entity_button.png

src/doc/tutorials/img/edit_mode_button.png

src/doc/tutorials/img/screenshot_dropdown.png

src/doc/tutorials/img/screenshot_edit_create_delete.png

src/doc/tutorials/img/screenshot_menu.png

src/doc/tutorials/img/screenshot_new_recordtype.png

src/doc/tutorials/img/simple_data_model.png

src/doc/tutorials/img/simple_data_model.svg

src/doc/tutorials/img/simple_data_model.xmi

src/doc/tutorials/img/very_simple_data_model.png

src/doc/tutorials/img/very_simple_data_model.svg