Remove Units and periodic tables

[matt-chan]

There are only a few cases where units and periodic tables are needed (IO/glue-code and gbasis/grid respectively), so it is not worth breaking modularity for it.

We need to replace those situations with simple dictionaries of data (atomic-nubmer:covalent-radii for grid, symbol:atomic-number for gbasis). There is a little bit of duplication of data, but it's a small amount that will never get changed. The convenience of having a periodic table class should be reserved for glue-code level or maybe even user code.

[tovrstra]

I would not remove them entirely. Units can be part of iodata. A periodic table package would still make sense as an independent project because it comes in handy when scripting and glueing, e.g. when some sort of radii is needed.