Store statistics in questions (and exam)

[dbosk]

Read back the exam, add statistics to each question and write it back to disk. We can also do the same for the exam as a whole.

This way we should be able to keep statistics similarly to examtool.

We should add a separate tool that handles the statistics: \eg examgen for generating the exam and examstats to handle the statistics and follow up. Then maybe rename the repo to examtools.