Pytorch Geometric support (deprecates KGCN)

[jmsfltchr]

What is the goal of this PR?

Migrate the ethos of TypeDB-ML to be a library to support machine learning using existing graph learning frameworks, starting with support for PyTorch Geometric and NetworkX to support in-memory graphs and general-purpose graph algorithms.

What are the changes implemented in this PR?

• Encoding of a TypeDB graph (already exported into NetworkX) into a PyTorch Geometric HeteroData object
• Updating the Diagnosis example to demonstrate how to create a GNN using TypeDB, TypeDB-ML and PyTorch Geometric
• Refactoring of the project structure
• Many minor build updates

Solves #44

[lolski]

Approved for the build only. The API contains many changes and most of them are fundamental. They will be reviewed later in a longer discussion (most probably in person) with James and Joshua.