1) Adding Inverse.java which creates the inverse of a given data graph.
It is useful to dump parentIndex for future usage.
2) Adding a piece of code for arranging ID of vertices between their min
and max. It is useful to reduce memory cost when some of IDs are not
used in the original data graph.
3) Arrangin the ID of the vertices in an extracted query from 0 to max.
For now, not writing the extracted subgraph with original IDs.
4) Small changes in buildParentIndex() and pre() in Graph.java