Adding RRFS Ensemble Mean/Recenter capability into HAFS

[XuLu-NOAA]

Description

Provide a clear and concise description of the requested feature/capability.
Current ensemble mean/recenter capability in HAFS is based on the GSI/enkf code, which is not recommended and is soon to be replaced by JEDI.

Proposed solution

How should the new feature/capability be added? If you have thoughts on the implementation strategy, please share them here.
Add the Ensemble mean/recenter code from RRFS into the HAFS DA system. The RRFS mean/recenter code has the parallel configuration that also works more efficient than the current GSI-based codes.

Status (optional)

Do you (or a colleague) plan to work on adding this feature?
@XuLu-NOAA will be working on the implementation.

Related to (optional)

Directly reference any related issues or PRs in this or other repositories, and describe how they are related. Examples:

• fixed by hafs-community/hafs/pull/<pr_number>