Granger causality tutorial notebook

[rjurkus]

I have implemented the GC tutorial notebook focusing on pairwise and conditional causality methods. Please feel free to suggest changes and additions.

[coveralls]

Coverage remained the same at 90.252% when pulling e08556a on INM-6:feature/ar_granger_tutorial into 25d4b1e on NeuralEnsemble:master.

[dizcza]

Built tutorial: https://elephant-docs-development.readthedocs.io/en/feature-ar_granger_tutorial/tutorials/granger_causality.html

[minor] There is an issue with the diagram plot (check the built doc), that is circles are (partially or completely) invisible. I'm thinking about whether we should create a folder with such images for additional materials. For now, just fix the diagram.

A description that the direct and indirect non-zero causality values are expected at the end of the tutorial is missing. Probably, you wanted to show that in this case a non-zero value is expected (but the value itself does not matter unless we want to compare it with another causality value).