Improve degree and nb_edges

[amontoison]

• Specialize degreeand nb_edges if we have loops or not.
• Don't check all values in degree if it's not needed.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 99.75%. Comparing base (848a6f3) to head (3a39e63).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/graph.jl	73.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #211      +/-   ##
===========================================
- Coverage   100.00%   99.75%   -0.25%     
===========================================
  Files           14       14              
  Lines         1638     1647       +9     
===========================================
+ Hits          1638     1643       +5     
- Misses           0        4       +4     

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[gdalle]

I think we're missing coverage because our default test suite for bicoloring only uses the random order, which never computes degrees.
We might want to test in the random suite with LargestFirst() too?

[amontoison]

Yes, good idea!

[amontoison]

Simplified and added in #214