[MNT] - Minor updates & fixes

[TomDonoghue]

Minor updates & fixes, including:

• updating report strings & group plots for variable numbers of metrics
• update & generalize the gen_methods_txt_str function

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.59%. Comparing base (89ad00c) to head (12564c2).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
specparam/plts/group.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #374      +/-   ##
==========================================
- Coverage   97.61%   97.59%   -0.03%     
==========================================
  Files         152      152              
  Lines        6455     6476      +21     
==========================================
+ Hits         6301     6320      +19     
- Misses        154      156       +2     

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