breaking-change: Report traversed edges instead of vertices

[bobluppes]

This PR addresses #63.

Instead of reporting the traversed vertices in breadth_first_traverse and depth_first_traverse, we report the traversed edges.

In a follow up I would make a similar change to the shortest path algorithms, as a path given by a range of edges is more usable. See our current shortest path example.

[bobluppes]

FYI @Hromz

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (8348b33) 99.81% compared to head (ec33941) 99.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   99.81%   99.81%   -0.01%     
==========================================
  Files          22       22              
  Lines        1081     1076       -5     
==========================================
- Hits         1079     1074       -5     
  Misses          2        2              

Files Changed	Coverage Δ
include/graaflib/algorithm/graph_traversal.tpp	100.00% <100.00%> (ø)
test/graaflib/algorithm/graph_traversal_test.cpp	100.00% <100.00%> (ø)

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