Parses a rubric YAML file into the rubric list structure used by
rubric_import() and write_rubric_yaml(). Item order in the file
determines display order and keyboard hotkeys: the first ten items of a
question get the keys 1-9 and 0, and any further items have no hotkey and
are toggled by clicking in mark(). There is no limit on the number of
items. A group entry in the items list (format "1.1") is flattened: its
members join the question's item list in place, each with its group
set, and the group itself is recorded in the question's groups.
Validation is performed by the S7 constructors as items and groups are
built.