Implement a prototype algorithm to map a de Bruijn graph to a partial order graph

[xingjianleng]

• [x] The de Bruijn graph implementation online will consider the reverse complementary sequences. They hide the alignment relationship between two sequences. So, there's the need to implement a de Burijn graph construction algorithm (without reverse complementary sequences).
• [x] After the de Bruijn graph algorithm is implemented, a mapper from the de Bruijn graph to partial order graph is needed. Currently, we may focus on the partial order graph implementation by others or implement a basic partial order graph data structure.
• [x] Write some unit tests using pytest to “guarantee” the correctness of implementation.