To add GCN to our library

[wangzhen263]

Description

Graph Convolutional Networks (GCNs) is a recently proposed architecture which has had success in semi-supervised learning on graph-structured data. It is an important member of graph researches. I will port GCN into our stellargraph library.

Dataset will be Cora at this moment.

Done Checklist

• [x] Produced code for required functionality
• [x] Branch and Pull Request build on CI
• [x] Branch and Pull Request pass unit tests on CI
• [ ] Peer Code Review Performed
• [ ] Code well commented
• [ ] Documentation in repo

[youph]

Check out the keras implementation of GCN by Kipf: https://github.com/tkipf/keras-gcn

[youph]

@wangzhen263 is this still in "ToDo" status?

[wangzhen263]

PR https://github.com/stellargraph/stellargraph/pull/318