Add specific functions to make it easier to work with paths

[peterboncz]

This should start and be defined when the system stably works and we start testing it for trajectory storage & analysis applications.

For example, if we want to select paths from a repo of stored paths that pass through two nodes, cut them out and then feed them into GROUP BY paths we would at least need:

• path_intersects(path,vertex) : bool
• path_cut(path,vertex,vertex) : path