Export esemble members to cf-compliant netcdf

[s6sebusc]

Description

If more than one member is present, a new coordinate "mem" is added for the members.

Issue Number

Closes #2246

[s6sebusc]

I know that the coordinates are originally stored in the config but I wasn't sure how best to add another coordinate that is only sometimes present. Another option would be to add the ensemble coordinate in every case and treat deterministic runs as single member ensembles. That might be convenient for some applications, but annoying for others.

[s6sebusc]

Had to touch the coordinate ordering thing as well. Moved that part to Regridder.regrid_ds in hopes that it is more reusable there, shouldn't really change anything but I can also move it back to the specific gauss_to_regular thing if that's better.
I didn't know at which place in the coordinate order mem should go so I put it last (also any other future coordinates with be appended after that in the current implementation), let me know if there is some preferred place for it.

[enssow]

Hi Sebastian, thank you for working on this I have some suggestions contained within the PR i opened :)

Tested with q76cdspy which only has the tp_imerg_0 valriable for stream IMERG_ANEMOI and ens members. If exported using --format netcdf it returns the ensemble members as the coordinate mem with CF-compliant attributes.

Tested with ryjwbxus which has all variables for stream ERA5 (no ens members). If exported with --format netcdf or --format verif returns the same netcdf as before.

It would be useful to test with a run_id that has ensemble members for ERA5 to check the knock on effect on verif_parser but I will open a sepearte issue for this when the time comes

[iluise]

Thanks for testing! you can use for example ege1pq8v on Santis (4 ensemble members). Do you have access?
If not, @wael-mika has IMERG run with 16 ensemble members on Jupiter. Wael can you share the run_id?