FIX:annotation semantics: adresses #1283

I followed the logic of the discussion in #1283: changed the names of read_annot write_annot to match the pattern we established. I also changed _read_annot to read_annot (and did the same for _write_annot).
Hm, I also prefer consistent ordering as in `read_annot_labels` and `write_annot_labels`, I don't think people covertly "fill in" that preposition in `read_annot_labels` and `write_labels_annot` (I don't). You'd have to spell out `read_annot_as_labels` and `write_labels_to_annot` for that to make sense (But couldn't you just as well say  `read_annot_as_labels` and `write_annot_from_labels`?)