Support the correction of ipynb file

[barais]

The goal is to use correctexam to support the correction of a set of ipynb file with the same kind of feature. Positive, negative or direct grading. Support of comments, statistics ....

[barais]

Among the component we have to test https://github.com/jirutka/ipynb2html

We could use cell metadata at the beginning to identify the question and get questions metadatas (question label, max score, step, type of questions).