#227 CIF tutorial input variables lesson: add SVG interaction example.

• I've added to the CIF tutorial lesson on input variables an example of using input variables with SVG interaction, by means of SVG input mappings with updates. These concepts are explained only later in the tutorial. Still, it is good to have all the examples together, even if users may not yet understand all the details. It is explicitly mentioned that they learn the details later. And in that later lesson, there is already a reference back to the input variables lesson.

Addresses #227 (closed)