Database keys for multi-species calculators

[PythonFZ]

Describe the bug
Currently there is adf_data["O_H_H"] to give access to an ADF with the central atom being O and two neighbouring H.
Maybe rename it "O_(H_H)" to highlight the central atom.
For a System of three species we should ensure that there is some given order e.g. for ["N-O-H"] vs ['N-H-O'].

For RDF there is no dependency on the order so "O-H" is identical to "H-O"