DAGify of graphs to convert paths into slices

[6br]

DAGify is the first step to convert paths into slices for graphs that we cannot assume that the graphs are already directed acyclic graph.

[6br]

There are procedures in visualizing graphs:

1. Retrieve a subgraph from the whole graph by tracing the nodes from the initial node. (I have not implemented yet because I think it is enough to use vg chunk command in MVP, but we need to implement the algorithm in future, whose algorithm is as written by Josiah.)
2. DAGify nodes to determine the order of nodes from the input. First, select the first path as a reference path, and then other paths are compared to the reference path iteratively to capture the node that shared in multiple paths using a longest common substrings DP. Each node between those common nodes should belong to the same slice. -> This is implemented in #4: Add DAGify method for linearizing the order of nodes #9.
3. Align the order of paths (For example, path(x,y,z) might be displayed as [y,x,z] by comparing the relevancy of paths), but it has not implemented yet. To compare the relevancy of paths, create a big bloom filter to store the hash(node_id). The hamming distance of bloom filter may be an estimated distance between paths. Considering the similarity distance, we can build "guide-tree" from the similarity ratio to determine how to cluster these paths.