Source label maps

[ndilalla]

Implemented the feature to label only a single source (or a list of sources) in TS and residual maps (issue #623 )

• Added label_source parameter that accepts a string (single source) or a list of source names
• When label_source is provided, only those sources are labeled, overriding the label_ts_threshold logic
• If label_source is not specified, the existing TS threshold behavior is used

[codecov]

Codecov Report

❌ Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.02%. Comparing base (92cf94f) to head (cb63491).
⚠️ Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
fermipy/config.py	75.00%	1 Missing ⚠️
fermipy/plotting.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #657      +/-   ##
==========================================
+ Coverage   50.79%   51.02%   +0.23%     
==========================================
  Files         143      144       +1     
  Lines       27265    27359      +94     
==========================================
+ Hits        13849    13961     +112     
+ Misses      13416    13398      -18     

Flag	Coverage Δ
unittests	51.02% <95.91%> (+0.23%)	⬆️

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