Add periodic table data

[matterhorn103]

Data such as:

• Standard atomic weights
• Mass numbers
• Relative isotopic masses
• Half-lives?

Given the volume of data this would be, some sort of container class or namespace module would be a good idea.

At the same time, given the existence of many other packages that provide this data (chemlib, periodictable, ChemFormula, mendeleev, chempy...) it would be worth considering how much it's sensible to duplicate work that's already been done.

Another option would be to have one of those packages as an optional dependency and rely on it for the data, and just provide an interface that returns quanstants.Quantity objects.