lh5concat: allow concatenating of group-like structs

[ManuelHu]

"fixes" #119

this does not (yet) have a general fix as it only warns about a known "blacklist" of unmergeable data.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.30%. Comparing base (782f566) to head (0104086).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lgdo/cli.py	71.42%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   78.05%   78.30%   +0.25%     
==========================================
  Files          46       46              
  Lines        3368     3384      +16     
==========================================
+ Hits         2629     2650      +21     
+ Misses        739      734       -5     

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

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests