[Tile-serving] [openstreetmap/osm2pgsql] Allow propagating tags to ways during relations processing (#230)

[Christoph Hormann]

> I don't doubt that there is a need, I just contest that osm2pgsql is the right tool for all those uses.

You should be aware that this is not visible to the observer from the documentation AFAIK.  The Readme says:

> osm2pgsql is a tool for loading OpenStreetMap data into a PostgreSQL / PostGIS database suitable for applications like rendering into a map, geocoding with Nominatim, or general analysis.

I see in principle three possibilities how this pertains to relations:

* by generating geometries representing the relations in question.  This currently is the approach used for specific relation types (multipolygon and route).  It could be generalized to using geometrycollections for representing relations in general.  The problems with route relations however demonstrate that this approach has serious limitations.
* by generating non-geometry tables referencing the geometry data (this is what `planet_osm_rels` does).
* by propagating information of the relation to the member geometries.  This is what mostly has been discussed in this issue.

There have been statements that a viable implementation of the third option could be acceptable for osm2pgsql at least for route relations.  There is now a statement that the second option is not considered to be acceptable beyond 'abusing' existing functionality.  It would make sense to document this (and preferably also the reasoning behind it) somewhere where it can easily be found by users.

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