Add shortcut for calculating and plotting RDFs

[stefsmeets]

This PR makes it easier to calculate and plot RDFs.

rdf_data = transitions.radial_distribution(
    floating_specie='Li',
)

for rdfs in rdf_data.values():
    fig = rdfs.plot()
    fig.show()

Closes https://github.com/GEMDAT-repos/gemdat-notebooks/issues/12