Autoassigning points to polygons

We increasingly have GIS boundaries for administrative divisions, as
well as GPS coordinates for each health facility.

Since the administrative divisions in some places are often shifting,
but health facilities are mostly fixed in space (though they can of
course be shut down, relocated or upgraded to different type), it
makes sense for us to implement functionality for automatically assign
a point to the lowest administrative unit it falls within.

This kind of functionality would also be very useful for surveys where
GPS points are also collected. I think it should not be too hard to
implement (though maybe not very urgent), and have made a blueprint
for it here: