Simplify usage of std::optional<bool>

[vil02]

This change consists of two parts:

• use std::any_of and std::none_of in isDirectedGraph() and isUndirectedGraph() respectively,
• simplify the logic of using std::optional<bool> by mostly using .value_or().

I decided not to update the test files. My plan is to do in in another PR, so it will be easier to see that these changes do not change the logic.

[codecov]

Codecov Report

❌ Patch coverage is 92.68293% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.07%. Comparing base (e968842) to head (37779f6).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
include/CXXGraph/Graph/Algorithm/Dijkstra_impl.hpp	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
- Coverage   97.08%   97.07%   -0.01%     
==========================================
  Files          98       98              
  Lines       11355    11333      -22     
  Branches      799      796       -3     
==========================================
- Hits        11024    11002      -22     
  Misses        331      331              

Flag	Coverage Δ
unittests	97.07% <92.68%> (-0.01%)	⬇️

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