Fix spectra combination for Dyson approximations

[obackhouse]

This shold fix Spectral.combine in the case of Dyson approximations (where both sectors have identity overlap)

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.37%. Comparing base (abd4d56) to head (76caa9a).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
dyson/representations/spectral.py	77.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   72.38%   72.37%   -0.01%     
==========================================
  Files          38       38              
  Lines        3867     3873       +6     
  Branches      492      494       +2     
==========================================
+ Hits         2799     2803       +4     
  Misses        869      869              
- Partials      199      201       +2     

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