Add ST_ClosestPoint (formerly ST_NearestPoints)

This new H2GIS function is related to the old GDMS function as follows:
```mysql
ST_ClosestPoint(geomA, geomB) = ST_NearestPoints(geomB, geomA)
```
It returns the 2D point on `geomA` closest to geomB. Note that the GDMS
function `ST_ProjectTo` is just a special case:
```mysql
ST_ClosestPoint((multi)linestring, point) = ST_ProjectTo(point, (multi)linestring)
```
For this reason, we will not implement `ST_ProjectTo` in H2GIS.

*Note*: If the closest point on geomA is not unique (it is easy to
imagine an example where there are infinitely many, mathematically
speaking), `ST_ClosestPoint` returns the first one it finds when
traversing geomB's coordinates, so the point returned depends on the
ordering of geomB's coordinates.

TODO: Rename `ST_FurthestPoint` -> `ST_FurthestCoordinate`, since this
function returns coordinates and not points.

TODO: Implement `ST_ClosestCoordinate`, which will return the closest
coordinate (rather than the closest point). (By request of @gpetit)

TODO?: Add `ST_3DClosestPoint`? See
        http://www.postgis.org/docs/ST_3DClosestPoint.html