Review EdgeSet, NodeSet, CellPopulation files

[josephdviviano]

Use self-documenting style when possible, also all classes have docstrings.

[Bertinus]

@josephdviviano

There is now a simple example for defining a custom CellPopulation class, and simulating trajectories with it. (see

FLeCS/flecs/cell_population.py

Line 119 in d0f1147

if __name__ == '__main__':

)

I removed the cell_factory.py file as I moved all the logic directly into the CellPopulation class.