Nbautoeval

* cosmetic tweaks

* this version is ***not backward-compatible** with 0.6 and below
* rendering engine entirely rewritten to take advantage of ipywidgets
* as a consequence all 4 flavours of `Exercise` have a different
interface for tweaking how results are being shown up;
as an example, `ExerciseFunction` accepts a `call_renderer`
and a `result_renderer` attribute, that decide how to format
the call column (1st) and result columns (2nd and 3rd)
* new Quiz feature to define multiple-choice tests

* no new feature, just a minor cleanup before big rewrite

* feature for issue 9: an ExerciseClass instance can be created with check_init=False

* bugfix for issue 7

* essentially same features as 0.6.0, except for `copy_mode="tee"`
that remains an available option but only explicitly
as it breaks some of our gen-based exos