KeyError when EPR simulation is not sufficiently populated

[SmolakK]

In the EPR class in the models.epr module, the preferential_exploration() method returns a KeyError when performing a location search on the precomputed od_matrix. It is related to the case, when _starting_loc of agents are randomly generated. In some cases, the starting position of an agent might be chosen to be in a location where there were no data points, hence it is not present in the precomputed od_matrix.

Potential solutions are to:

1. Generate agent positions only for locations where any data in the precomputed od_matrix are present.
2. Limit spatial_tesselation to the locations where any data are available.
3. Force the user to provide starting locations on her own.