make quiz changes more straightforward

[parmentelat]

with 1.0 each time a quiz is changed on file, the kernel needs to be restarted to see the updated quiz

there are 2 levels of caching at play

• the python module does not get reloaded; %autoreload could help with that
• check if that's enough; there is some internal caching that preserves the widget if already created, which may be in the way

in any case, modifying a quiz is currently tedious and needs to be ironed out; ideally just re-running the quiz.widget() thing should do it

[parmentelat]

using yaml makes things much simpler since the Quiz object is rebuilt each time from scratch