Radii information

[schneiderfelipe]

It would be nice to have some data on atomic radii. There are actually many different ways of defining atomic radii (covalent, van der Waals, etc.); check out how the Python package mendeleev organizes this type of data. This is an important addition, as calculations employing atomic radii show up in many molecule manipulations.

[rahulkp220]

@schneiderfelipe Please feel free to open a PR 👍