R package documentation and tutorial update

public/rqgisapi/articles/tutorial.html

...api/articles/tutorial_files/figure-html/saveAsImage-1.png

public/rqgisapi/reference/iface.addRasterLayer.html

public/rqgisapi/reference/iface.addVectorLayer.html

public/rqgisapi/reference/index.html

public/rqgisapi/reference/mapCanvas.saveAsImage.html

public/rqgisapi/reference/mapLayer.getFeatures.html

...rqgisapi/reference/mapLayer.selectedFeatures.default.html

public/rqgisapi/reference/mapLayer.selectedFeatures.html

public/rqgisapi/reference/rqgisapi.html