Model selection

[VinsRR]

This branch proposes the following modifications:

• Implementation of model selection for MultiOrderModel (for walk data only)
• tests for the modules related to model selection in MultiOrderModel
• functions compute_weighted_outdegree and compute_transition_probabilities (necessary to perform model selection. These functions are located in MultiOrderModel.py
• fixing weights for higher-order networks (thus making the branch fix-hon-weights obsolete) #157

The modifications in this branch also relate to the following topics:

• Edge weight representation #155 -- workarounds have been developed to relate edge frequencies to path frequencies (as, currently, path frequencies are saved into edge frequencies). See all the frequencies variables in MultiOrderModel
• Zeroth order layer: workarounds have been developed to perform the model selection without needing the zeroth order in MultiOrderModel. The question remains whether we want to implement one or not #172