Add mover/copy stage

[jbusecke]

Towards #33 and will help keeping the ultimate target store 'clean' as in there will be no empty or half-written stores.

[jbusecke]

Ok some of the jobs (example) fail with

FileNotFoundError: [Errno 2] No such file or directory: 'gsutil' [while running 'Creating CMIP6.ScenarioMIP.MPI-M.MPI-ESM1-2-LR.ssp126.r16i1p1f1.Omon.o2.gn.v20210901|OpenURLWithFSSpec|OpenWithXarray|Preprocessor|StoreToZarr|MoveStore|Logging to non-QC table|TestDataset|Logging to QC table/MoveStore/Moving Zarr Store-ptransform-59']

what is the best way to get gustil on the runner? @yuvipanda @cisaacstern

If that is hard I might consider using the beam IO module... but would be nice if this simple stage would work...

[jbusecke]

Lets see if adding gustil to the requirements.txt just works...

[jbusecke]

So I am running into this issue when calling gusti via subprocess.run:

FIND THIS: ServiceException: 401 Anonymous caller does not have storage.objects.list access to the Google Cloud Storage bucket. Permission 'storage.objects.list' denied on resource (or it may not exist).

So we do not have the right permissions. How is access actually set within a Ptransform?

Wondering if @yuvipanda or @cisaacstern have an idea here.

Quick summary: I just want to copy the zarr store from a given path to another within a Ptransform

[jbusecke]

Ok so using beam.io worked quite well. Thanks to @sharkinsspatial for the initial push, and @cisaacstern for the pair programming today.