PoolAllocator: fix handling of first shape/dimension being a range index

[MichaelSt98]

We already handled range indices like e.g., 0:10 properly as long as those aren't the first dimension/shape ...

Thanks @piotrows for pointing this out!

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/482/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.17%. Comparing base (e79d1da) to head (b3f8b9f).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           nams-pool-allocator-improvements     #482   +/-   ##
=================================================================
  Coverage                             96.17%   96.17%           
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  Files                                   224      224           
  Lines                                 40336    40346   +10     
=================================================================
+ Hits                                  38793    38803   +10     
  Misses                                 1543     1543           

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	96.17% <100.00%> (+<0.01%)	⬆️

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