delete return lines when in calculate or aggregate in Models.py

In all models in Models.py, when the user calls name.calculate() the code returns the result which can get in the way especially when working with many origins and destinations. In the noteboooks, the results should only print if the user calls for it or uses head()

Start by reading the model calculation and aggregation implementations in Models.py, then inspect the notebooks that call them. The work is complete when calculate() and aggregate() no longer display results implicitly, while explicit output requests such as print or head() still show results.

Written by the indexing model from the issue text.