Fix cutout slicing of grid dimension

[japols]

This PR fixes a bug with slicing Cutout datasets where slicing the grid dimension didn't work:

cfg = {
    "cutout": [
        {
            "dataset": "metno-meps-archive-det-opendap-2p5km-2020-2023-6h-v1", 
            "thinning": 25
        },
        {
            "dataset": "aifs-od-an-oper-0001-mars-o96-2016-2023-6h-v6"
        }
    ], "adjust": "all"
}

ds = open_dataset(cfg)
sample = ds[1324:1327, :, :, :]
print(sample.shape)
sample = ds[1324:1327, :, :, 0:20000]
print(sample.shape)

previously gave

(3, 85, 1, 41459)
(3, 85, 1, 41459)

now gives

(3, 85, 1, 41459)
(3, 85, 1, 20000)

@b8raoult @floriankrb @JPXKQX @paulina-t

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.96%. Comparing base (2c8bd40) to head (3fbe0ed).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #145   +/-   ##
========================================
  Coverage    72.96%   72.96%           
========================================
  Files           10       10           
  Lines          825      825           
========================================
  Hits           602      602           
  Misses         223      223           

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[JPXKQX]

It looks good to me