add function to convert neo4j native point(s) to WKT format

[Andy2003]

This PR adds a function spatial.convert.nativeToWkt to convert native points to WKT:

return spatial.convert.nativeToWkt(point({longitude: 1, latitude: 2})) as wkt

returns:

POINT ( 1 2 )

---

return spatial.convert.nativeToWkt([point({longitude: 1, latitude: 2}), point({longitude: 3, latitude: 4}) ]) as wkt

returns:

LINESTRING (1 2, 3 4)

[craigtaverner]

Nice feature, but feels like we need to think of more consistent names.