Implement model cell bookmarking

Absolute references to input cells when calling `parseModel()` lead to problems if model cells move due to insertions / deletions; relative references are possible, _e.g._ `In[-2]` to refer to the preceding cell, but is not a general solution as insertions between the model and the relative reference will still cause problems. Ideally a means to bookmark a model cell and then reference it should be investigated, if possible within the Jupyter framework.