Add ensemble (letkf) option to run_jedi_exe.py

[RussTreadon-NOAA]

This issue is opened to document the modifications needed so that ush/run_jedi_exe.py can execute fv3jedi_letkf.x provided a suitably configured input yaml. The following tasks need to be completed:

• add low resolution ensemble to R2D2_SHARED database
• ensure r2d2 is able fetch member states from a central database and stage them in a unique run directory
• add ensemble as a GDASApp mode supported by run_jedi_exe.py
• update, as necessary, scripts in ush/ufsda to generate yaml for fv3jedi_letkf.x run using user input
• add yamls to examples/run_jedi_exe/ for ensemble DA on both Hera and Orion

The work in this issue benefits from the merger of PR #108 into develop. PR #108 allows variational and ensemble DA modes to use the same templated observation yamls.

[RussTreadon-NOAA]

Work for this issue will be done in branch feature/letkf. feature/letkf originated as copy of develop at 44f5abc.

[RussTreadon-NOAA]

Close this issue. Replaced by GDASApp issue #223