Cell type changes break the grading flow

[trevorcampbell]

Students often accidentally change the type of read-only cells (e.g. from markdown to raw). This causes nbgrader to break.

[trevorcampbell]

A good solution to this would be to check for any read-only cells in the source/release notebook, and overwrite the student's copy with the instructors copy

[trevorcampbell]

Just realized there's an even easier solution -- here's an example of a failed case:

{"cell_type": "raw", 
"metadata": {"deletable": false, 
                     "editable": false, 
                     "nbgrader": {"cell_type": "markdown",
                                         "checksum": "7b875f4e42e8f36bbf2ff82e266ac1b1", 
                                          "grade": false, 
                                         "grade_id": "cell-e7cd941c74d291b3", 
                                         "locked": true, 
                                         "schema_version": 3, 
                                         "solution": false}
                       }, 
"source": ["blah"]}

Basically I think in the cleaning step we can just check whether nb["cell_type"] == nb["metadata"]["nbgrader"]["cell_type"] and if not, replace the main cell_type with the nbgrader cell_type

I will fix that in prefect-by-assignment but will leave this open for rudaux-1.0-dev