Resolve "Proposal for adding other building datasets"

Closes #25 (closed)

\rfc @tara @ds

We may use a query to select the best building per quadkey, but I have another proposal that may have some advantages, with the main disadvantage being that the copying of buildings from postgis to spatialite is a bit more complex (but still ok):

• easy to implement and minimal change of code
• per country we can decide which datasets to use as input, including a priority list (that can very per country)
• we implementation is as how we intended it (check each quadkey on presence of buildings in OSM, Google, Microsoft in order of priority), while if we use an SQL query, we don't exactly do that and will miss a bunch of buildings. See example here:

If the yellow buildings are OpenStreetMap and the red are Google buildings, with a query we'd say that the Quadkey highlighted in blue has OpenStreetMap buildings (because there is some built area there). Therefore we won't process the Google buildings. In the case of this MR, we would process them.

(\approve @tara @ds )