Radii information

It would be nice to have some data on [atomic radii](https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)). 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](https://mendeleev.readthedocs.io/en/stable/data.html#id3). This is an important addition, as calculations employing atomic radii show up in many molecule manipulations.

Start by reviewing how PeriodicTable.jl currently organizes element data, then compare the linked mendeleev organization of covalent, van der Waals, and other atomic radii. Done means the relevant atomic-radius data is represented consistently for the elements and is usable by molecule-manipulation calculations.

Written by the indexing model from the issue text.