DB/Overpass: ways can no longer be area and line - depend on condition
DB/Overpass: check outer ways from multipolygons

make sure, that a way which was part of a multipolygon (with tags from
outer ways) is not returned as an area - it may still be a line
DB/Overpass: Multipolygons: relation parent may have same tags as children
DB/Overpass: improve multipolygon support

- set 'osm:has_outer_tags' to 'yes', when multipolygon has tags from
  outer ways
- when relation and outer ways have the same (relevant) tags, the
  structure comes from relation; the ways are no areas
- improve documentaiton
- remove debug message
Merge branch 'db-overpass' into separate-loading-ways

Conflicts:
	pgmapcss/db/overpass/db_functions.py
Merge pull request #88 from plepe/separate-loading-ways

DB/Overpass: Separate loading ways