Document species covered by each dataset

[gabrielasd]

As pointed out on issue #125, we should provide information on what species are available with each dataset.

... how we can easily express which atoms/ions are covered by which datasets. It would be good to have a very short description of which species are covered in each dataset, especially since only the hci dataset is actively changing.

[PaulWAyers]

@msricher pointed out that one can assess which species are present using

For the load function, you can use ... for any of the elem, charge, mult, nexc values.

We should probably add some usage of this in one of our example notebooks, so that people can understand this usage too.