GMPP: Add convenient main function

Blocked by #602.

Add GMPP main function that:
1. Takes GNN model type (like MPNN or GAT) and optionally some kwargs parameters for it.
2. Takes featurisers preset (for atoms and bonds).
3. Takes CSV dataset, SMILES column and Assay column.
4. Builds model, performs cross-validation and returns train, validation and test score.
5. Use it in the notebook.

Going extra mile:
* Add CLI interface using `argparse`.