plot/ssf: adjust to new structure of the H5MD files

The particle groups are given priority over the various observables.
This version can read files produces by HAL's MD package since commit
cf88f59.

old format:
    /structure/ssf/wavenumber
    /structure/ssf/A/B/value

new format:
    /structure/A/B/static_structure_factor/wavenumber
    /structure/A/B/static_structure_factor/value