Add a utility for generating cutoff radii naturally from the covalent radii

Merged username-removed-263547 requested to merge tgmaxson/ase:natural-cutoffs into master

Generating cutoff radii from covalent radii turns out to be a really common task for building neighborlists, so it would be helpful to add this as a utility for ASE. There are some options to override radii or just increase them by a percentage across the board as well, so it should fit most use cases.