Enh: Add CLI

[pckroon]

I think it would be a nice feature to add a simple CLI to pre-fill the input form, e.g. -f to provide a pdb file, -s for the smiles string, --keep-H to keep hydrogens. This would result in some duplication, so bonus points if we can autogenerate either the GUI form or the CLI from the other, or provide the inputs in a separate data structure somewhere, and generate both from that.

Note that QApplications also use CLI arguments (and may consume some), see also https://doc.qt.io/archives/qt-4.8/qapplication.html#QApplication. The good news is that it will consume the args it knows, and leave the rest available in qapp.arguments (https://doc.qt.io/qtforpython/considerations.html#qapp-macro).