kgcnn v3.1.0

• Added flexible charge for rdkit_xyz_to_mol as e.g. list.
• Added from_xyz to MolecularGraphRDKit .
• Started additional kgcnn.molecule.preprocessor module for graph preprocessors.
• BREAKING CHANGES: Renamed module kgcnn.layers.pooling to kgcnn.layers.aggr for better compatibility.
  However, kept legacy pooling module and all old ALIAS.
• Repair bug in RelationalMLP .
• HyperParameter is not verified on initialize anymore, just call hyper.verify().
• Moved losses from kgcnn.metrics.loss into separate modul kgcnn.losses to be more compatible with keras.
• Reworked training scripts especially to simplify command line arguments and strengthen hyperparameter.
• Started with potential keras-core port. Not yet tested or supported.
• Removed get_split_indices to make the graph indexing more consistent.
• Started with keras-core integration. Any code is WIP and not tested or working yet.