BFS and DFS with unique allocator

[schaumb]

At BFS and DFS base, the three_color vectors are not follow the passed Allocator type.

https://github.com/stdgraph/graph-v2/blob/master/include/graph/views/breadth_first_search.hpp#L186 https://github.com/stdgraph/graph-v2/blob/master/include/graph/views/breadth_first_search.hpp#L72

So if I want to use my own allocator, like std::pmr::monotonic_buffer_resource, I got compile error.