refactor: algorithm directory structure

[bobluppes]

While the diff of this PR is massive, it does not contain any logic changes. Instead, it merely splits source files and moves them around.

• Each algorithm is now in its own source file. Same for the corresponding tests.
• Algorithm directories consistently use an _ rather than -

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
.../algorithm/cycle_detection/dfs_cycle_detection.tpp	95.12% <ø> (ø)
...algorithm/graph_traversal/breadth_first_search.tpp	100.00% <100.00%> (ø)
...nclude/graaflib/algorithm/graph_traversal/common.h	100.00% <100.00%> (ø)
...b/algorithm/graph_traversal/depth_first_search.tpp	100.00% <ø> (ø)
...aaflib/algorithm/minimum_spanning_tree/kruskal.tpp	100.00% <ø> (ø)
...nclude/graaflib/algorithm/shortest_path/a_star.tpp	100.00% <100.00%> (ø)
.../graaflib/algorithm/shortest_path/bellman_ford.tpp	100.00% <100.00%> (ø)
...flib/algorithm/shortest_path/bfs_shortest_path.tpp	100.00% <100.00%> (ø)
include/graaflib/algorithm/shortest_path/common.h	100.00% <100.00%> (ø)
...nclude/graaflib/algorithm/shortest_path/common.tpp	100.00% <100.00%> (ø)
... and 19 more

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