subgroups=False seems fundamentally broken

I thought I had this thought through, but in my refactoring with DotDict I somehow broke the subgroups=False option. I do not know yet what the correct fix is, because I want the implemenation of the meta ini expander to be as slim and elegant as possible (not cluttered by ifs that check for nestedness of the dict). Additionally, this has a low priority as we do not use this in Dune/Dumux. It is only relevant to other projects (and even then, they could just escape the dots in the key names).