incorporate HPV status

[rmnldwg]

HPV (human papilloma virus) status is widely considered an important prognostic factor in oropharyngeal SCC. Thus, our model should be able to factor it into its predictions. However, we do not yet have a good idea how to do that.

One idea is to allow HPV+ and HPV- patients to have slightly different "time priors", indicating that not the patterns of lymphatic progression, but their speed is different for HPV+ patients.

[YoelPH]

Just for the record: An implementation failed to properly model HPV spread by adapting the time priors for HPV positive patients. The main reason is that the data does not really imply a faster spread to all LNLs, but mostly to LNL II.
A model with a different base parameter from tumor to LNL II was more successful in modelling the HPV positive patients. The downside is, that for such a model no straight forward anatomical reasoning can be found.