Implement model cell bookmarking

[jarmarshall]

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.