Update atomic_density.ipynb

[FarnazH]

This PR is meant to finalize the atomic_density.ipynb notebook.

@Ali-Tehrani can you please include instructions on how to obtain the Slater density files?

[FarnazH]

P.S.: @Ali-Tehrani I noticed that the data exists in https://github.com/QuantumElephant/bfit/tree/master/bfit/data/data.tar. We should add comments on untaring data.tar.

[Ali-Tehrani]

P.S.: @Ali-Tehrani I noticed that the data exists in https://github.com/QuantumElephant/bfit/tree/master/bfit/data/data.tar. We should add comments on untaring data.tar.

I can't get your remote to push to this and github doesn't let me edit, we can add under step two " Compute Slater Density" the following:
"The data.tar folder in ./bfit/data/ folder needs to be untar, inside ./bfit/data/ folder in order to compute the atomic density for various atoms. See the [installation](https://github.com/theochem/BFit/) instructions in the README for more details."