Cube object housekeeping

[Stiivi]

Housekeeping changes to the Cube object:

• [ ] remove aggregates_for_measure(name)
• [ ] remove all_attributes()
• [ ] reconsider use of attribute(name)
• [ ] try to replace the above with cube.get_attributes(...)
• [ ] #401
• [ ] link_dimension is mutable method, remove – require that in __init__
• [ ] rename distilled_hierarchies to something that matches the description
• [ ] change the return type of distilled_hierarchies to something simpler than Dict[Tuple[str,Optional[str]],List[str]]
• [ ] expand_dimension_links - own type DimensionLink

All method removals should make sure that the callers are modified as well.