Make radius configurable

The radius of the rendered atoms is hardcoded, making it configurable as a property in the UI (the import panel) would be a good idea to adjust the density of the visualization.

Relevant line: https://github.com/AndrewRadev/protein-runway/blob/bbd2f7840d8e612c0c956e442e45c67eccd5e7c1/blender/extension/ui/import_pdb_operator.py/#L75