Reduce duplication in the code that moves content from unit and groups (and back) #58
Description
Across the library, there are many points where content is moved from a unit to a group and other way around.
This moves its metadata and segments from one to the other.
There are small differences between all the instances of the code, but better to find a common way of dealing with those, and refactor to just on method instead of 4 or more