Add medulloPackage as analysis method

[envest]

https://github.com/d3b-center/medullo-classifier-package can now work as a single sample predictor -- we need to include it as a comparison method and understand its functionality

Once Single Sample Classification PR (their # 6) is merged:

• implement test_medullo() in utils/modeling.R with run_one_model() and run_many_models()
• update documentation throughout repo
• update each notebook to include medullo comparison

Rathi, K. S. et al. A transcriptome-based classifier to determine molecular subtypes in medulloblastoma. PLoS Comput. Biol. 16, e1008263 (2020)

[envest]

One concern about using this method is training data leakage. Many (all?) of the data sets used to train medullo also appear in our study. We need to be intentional about limiting our medullo testing to data sets not appearing in their training set. This may take the form of a distinct test analysis on particular data sets that don't overlap with their study.