- Implement a new constructor for `domain_decomposition` from a vector of `group_descriptions`. The new constructor performs various checks on the given `group_descriptions` and throws an appropriate exception if any problem is found. It is the only available constructor for `domain_decomposition`.
- Make the various members of `domain_decomposition` private and implement their getters.  
- Expose the `domain_decomposition` constructor to the python interface through a `partition_by_group` function.
- Add unit tests. 
- Add documentation.