Fixes #1551 – DataTree-native I/O for EchoData serialization

[beatfactor]

Replace the manual per-group save loop in _save_groups_to_file with DataTree.to_netcdf/to_zarr.

Changes:

• Nested encoding dict keyed by group path (e.g. '/', '/Environment')
• Object->str dtype sanitization before encoding
• Dask chunks aligned with encoding chunks for zarr parallel writes
• write_inherited_coords=True preserves backward compatibility
• 12 unit tests for roundtrip, dtypes, coord inheritance, dask arrays

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.49%. Comparing base (6cf6cee) to head (66dfd48).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
echopype/convert/api.py	94.28%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1635      +/-   ##
==========================================
- Coverage   85.58%   85.49%   -0.09%     
==========================================
  Files          79       79              
  Lines        6998     7018      +20     
==========================================
+ Hits         5989     6000      +11     
- Misses       1009     1018       +9     

Flag	Coverage Δ
integration	80.52% <88.57%> (-0.12%)	⬇️
unit	60.64% <91.42%> (+0.22%)	⬆️
unittests	85.39% <94.28%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.