Use the Zarr's new getitems() API

[madsbk]

By using the new API in zarr-developers/zarr-python#1131, we do not have to guess whether to read into host or device memory. That is, no more filtering of specify keys like:

 if os.path.basename(fn) in [ 
     zarr.storage.array_meta_key, 
     zarr.storage.group_meta_key, 
     zarr.storage.attrs_key, 
 ]: 

Notice, this PR is on hold until Zarr v2.15 is released

Closes #119

UPDATE: Zarr v2.15 has been released

[wence-]

Thanks @madsbk, I think we can do a little bit of cleanup, to avoid repeating ourselves with the minimum supported version.

[wence-]

The idiomatic way to do this is...

import contextlib

...
with contextlib.ExitStack() as stack:
    for key in keys:
        f = stack.enter_context(kvikio.CuFile(filepath, "r"))
        ret[key] = ..
        io_results.append((f.pread(ret[key]), nbytes)
   for fut, nbytes in io_results:
        nbytes_read = fut.get()
        ...

[wence-]

I'll note that both the new code (and my proposed approach) leave dangling futures if the block produces an exception anywhere. I guess that is fine because that propagates and the program is hosed at that point anyway?

[madsbk]

I think that is ok

[madsbk]

Thanks @wence-, I think I have addressed all of your issues.

[wence-]

Thanks Mads.

[madsbk]

/merge

[wence-]

I think we need to wait for the other codeowners to approve too.

[jakirkham]

Thanks all! 🙏

Great work 🥳