Refactor GroupMetadata{V3,V2} constructors

[LDeakin]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 10.25641% with 35 lines in your changes missing coverage. Please review.

Project coverage is 81.95%. Comparing base (be1727e) to head (b204ef3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs_metadata/src/v2/group.rs	0.00%	21 Missing ⚠️
zarrs_metadata/src/v3/group.rs	26.66%	11 Missing ⚠️
zarrs_metadata/src/v2_to_v3.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   82.06%   81.95%   -0.12%     
==========================================
  Files         166      166              
  Lines       23387    23415      +28     
==========================================
- Hits        19193    19190       -3     
- Misses       4194     4225      +31     

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