Polishing 3 : railway stations, roads at z9-10, access private

[Florimondable]

Code cleaning.
fix #272 and #262
fix almost #255 but I'm not able to render tertiary (WTF?)

[Florimondable]

Add allotments, wetland and some naturals (glacier is still missing at least) (for #229)
Should fix #263 (tourism point of view in brown) and #265 (allotments)

[Florimondable]

fix #259 private access roads in grey

[Phyks]

but I'm not able to render tertiary (WTF?)

roads_med layer is using planet_osm_roads table (not planet_osm_line table as used on upper zoom levels). This table only contains a subset of roads, which are useful at low zoom level (for perf), see https://gis.stackexchange.com/questions/37099/in-osm2pgsql-how-is-the-planet-osm-roads-table-populated.

In particular, it only has very few tertiary. The logic is basically described in https://github.com/openstreetmap/osm2pgsql/blob/master/style.lua (we are not exactly using this, but the core ideas of the logic are the same, check for the roads items). The few tertiaries listed in planet_osm_roads table are coming from artifacts from other items being added into this table (administrative boundaries along a tertiary for instance).

I'd expect the tertiary to have too much clutter at Z9/Z10, I would be in favor or removing them from roads_med layer.

[Phyks]

Few comments above, once settled I'm ready to merge this PR and publish a new version of the CyclOSM style :)