GEOSNearestPoints: returns mixed dimensions from 3D inputs

[Mike Taves]

See ​https://github.com/Toblerity/Shapely/pull/1006#issuecomment-787537048 for background

GEOSNearestPoints with GEOS <=3.9 returns only 2D geometries, regardless of inputs.

The current GEOS development snapshot may return mixed dimensions with 3D inputs. For instance, a PointZ and LineStringZ returns a PointZ and 2D Point object for nearest locations.

Is the expected result to maintain 2D-only geometries (i.e. GEOS<=3.9 behaviour)? Or should it be improved to interpolate the Z coordinate from the LineStringZ object?

Another case to consider is the nearest points between two PointZ objects. The current result is two PointZ objects.

[robe]

Retargeting in prep for GEOS 3.10.0 release