process simple multipolygons the same way as regular ones

… by consolidating two separate code branches. This makes sure
simple mps get the same amount of care as regular ones. fixes #13

also: do not unnecessarily produce MultiPolygons, when a Polygon is
enough to describe the geometry.
add test case for invalid simple multipolygons
add test for role-less multipolygon members

these are expected to be handled as outer-ways.