raster_to_vector creates artifacts due to tile regridding

[JeroenVerstraelen]

Before converting the raster to vectors, the raster_to_vector method regrids the rdd to become one single tile (or multiple very large ones) if the rdd is too big:
https://github.com/Open-EO/openeo-geotrellis-extensions/blob/master/openeo-geotrellis/src/main/scala/org/openeo/geotrellis/OpenEOProcesses.scala#L482

When raster_to_vector is used in the parcel delineation notebook it appears that this regridding causes artifacts to appear.

Input:

Output:

[jdries]

I pushed a simple fix and improved the unit test.
Still I noticed that vectorization did not seem to include the full original raster for some reason.
Run the unit test modified in this issue to observe current behaviour!

[jdries]

Proposed solution: add some overlap to the tiles, then vectorize, then perform a union that merges overlapping polygons