[Enhancement] Documentation/demos should probably cover some more use-cases in the future

[avohns]

Firstly, let me say that I really admire the work you have done with Numbas and this extension in particular.
Reading both the Numbas documentation and the readme provided here, it took me quite some time to figure out some things I think would justify an inclusion somewhere in the documentation:

1. I struggled to find out where I can actually set the amount of marks for a question of the type extension (because these lack the "Marking settings"-option). We can use set_credits() under "Marking algorithm" in our custom marking algorithm, but I found no way to comfortably set the marks-parameter itself. What I did was include this.marks=4 in the "When the part is created" field under "Scripts" -- which works just fine but is a bit oblique/should be documented somewhere.
2. As far as I can tell, none of the demos provided here cover the creation of new objects by the students, which was something I needed in my example and for which I had to figure out how to use the exists( )-function properly. Maybe one such example should be included as well? (I ended up using something similiar to if(exists(app,"f")=true,if(definition_string(app,"f")=f_correct, ...), ...); - which works if you know what the name of the next object that will be created by the user is (in this case f). I still have not figured out how to do it without knowing that.