Add ST_ShortestPathTree

Adds a new function to comput the shortest path "tree" (in quotes because there can be multiple shortest paths if they have the same length) from a given start node:
```mysql
SELECT * FROM ST_SHORTESTPATHTREE(nantes_WD.edges, 362, 'weight', 'directed - edge_orientation');
```

The search may be limited by a radius (say, 5km):
```mysql
SELECT * FROM ST_SHORTESTPATHTREE(nantes_WD.edges, 362, 5000.0, 'weight', 'directed - edge_orientation');
```

The notation is the same as ST_ShortestPath(Length).