Major refactor of the data package (vertices).
Merge pull request #29 from agouge/refactor/vertices

Major refactor of the data package (vertices).

The different vertices needed a redo to make their inheritance structure more better correspond to the corresponding structure of the algorithms. The new structure is as follows:

```
              VId
               ^
               |
<<VDist>>    VPred            <<VCent>>
    ^ ^     ^  ^ ^ ^               ^
    |  \   /   | | |               |
    |   \ /    | | |               |
    |    \     | |  -------------- |
    |  /  \    |  -------         ||
    | /    \   |         |        ||
VDijkstra    VBFS      VDFS    VCentImpl
    ^          ^         ^
    |          |         |
 VWCent     VUCent   VStrahler
```

Note that VWCent and VUCent have a local VCentImpl to which they delegate all centrality operations.