simple discovery of Protein-ligand binding AFFinity prediction

it includes:

• PAFNUCY https://gitlab.com/cheminfIBB/pafnucy preprocessing, training, predicting
• ECIF https://github.com/DIFACQUIM/ECIF preprocessing, training, predicting
• IGN https://github.com/zjujdj/InteractionGraphNet/tree/master preprocessing, training

and some utilities and helpers, like chimera-related scripts

following is slowly in progress:

• ECIF plus: working on PDBBind 2020; minimized and end2end ECIF workflow; add "escape" type that stands for all unsupported atom types