Implement summarize workflow for the exercise marking mode.

[remochristen]

This depends on #47 being merged.

Here is a suggestion on how this could work:

• Tutor 1 and 2 each do their corrections
• Both run collect, which creates a share_archive, this should include the individual points file
• Tutor 2 sends the share_archive to Tutor 1
• Tutor 1 stores Tutor 2's share_archive next to her/his own
• Tutor 1 runs combine, which should move the individual point files from both his own and Tutor 2's share_archive to some predefined directory where they are collected
• Tutor 1 should then be able to run summary without having to provide a path, as we know where the individual point files are moved

The idea is that whenever we use this mode, the assistant is usually not involved in keeping track of all the points, instead the tutors organize themselves. One caveat is that only Tutor 1 has the necessary information to create the report, but if needed the directory with all the individual point files can be shared of course.