Remove implicit group support

[LDeakin]

Pending zarr-developers/zarr-specs#292

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.32%. Comparing base (489f357) to head (e48c947).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/group.rs	75.00%	1 Missing ⚠️
src/node.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   82.28%   82.32%   +0.03%     
==========================================
  Files         160      160              
  Lines       22228    22206      -22     
==========================================
- Hits        18291    18280      -11     
+ Misses       3937     3926      -11     

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