Use PeriodicTable without instantiating

[pfebrer]

Hi Nick, is there a reason why you need to instatiate the PeriodicTable class to get the information? Not a big deal, but seems strange :)

[tfrederiksen]

I have been wondering about this too.

[zerothi]

I agree, this class needs an overhaul.

Actually I think the dispatch method could be useful here since users could register their own values on par with the default values!
Then something like

PeriodicTable.radius.vdw(...)

could be used. What do you think about such an interface?

[pfebrer]

Seems good to me! As long as you can still call the default with:

PeriodicTable.radius()

The only problem that I see is that maybe the documentation becomes more confusing. But maybe you know how to make it clear.

PS: Can a bond method be useful? I know it is very easy to get anyway by doing radii(["C", "H"]).sum(), but I don't know, it may be handy and it might make code more explicit.