Implemented multi sub-key support

Old getter are marked as deprecated. They will be removed when all other
methods have been changed to the newly returned Lists.

As discussed with @f-porter

Closes #206
Okay, I just wondered if are further, conceptional reasons.
Of cause, the memory consumption of this two extra lists is negligible.
My concern, if any, was about the redundant maintenance effort, but if you @L-Henke don't see any problems, then I shall be okay with it, too.