Create co_bfs.hpp & co_bfs_tests.cpp to explore coroutines

[pratzl]

Move code out of previous bfs code files
Disable use of boost::cobalt in favor of simple generator impl

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 97.87%. Comparing base (8052151) to head (9b220ac).
Report is 2 commits behind head on master.

Files	Patch %	Lines
tests/co_bfs_tests.cpp	77.14%	8 Missing ⚠️
include/graph/views/co_bfs.hpp	87.87%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   98.14%   97.87%   -0.28%     
==========================================
  Files          36       38       +2     
  Lines        3888     3955      +67     
==========================================
+ Hits         3816     3871      +55     
- Misses         72       84      +12     

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