Update case study: probabilistic matrix factorization (#152)

* [WIP] Updating the case study: probabilistic matrix factorization for personalized recommendations to use numpy.Generator

update pd.read_csv to pass arguments as kwargs

* add coords/dims, use xarray, add notebook style guide changes

* changes: OriolAbril

* referencing corrections

* toggle cell outputs

* cell tags to toggle output

Author: chiral-carbon