Refactor radial_distribution_between_species()

[stefsmeets]

This PR follows up on #338 , and makes it easier to access the rdf data. It also re-uses the rdf plotting code.

Hi @stefsmeets thanks a lot!
The only concern is that raw data is not easily accessible now, and it's nice to have such opportunity as sometimes we need to visualize several RDFs at one plot. Can we maybe move _get_radial_distribution_between_species function as a Trajectory function?

Originally posted by @AstyLavrinenko in #338 (comment)

Intended usage:

rdf = trajectory.radial_distribution_between_species(specie_1=..., specie_2=...)
rdf.plot()

# data is accessible via
rdf.x
rdf.y