Fix area generation from multipolygon input In case the waterway axis representing a stretch intersects distinct parts of the input area, a multipolygon has to be returned. Before, an arbitrary polygon was selected due to the used function not being set-returning. In passing, consistently name respective columns in schema.

teichmann@intevation.de = sascha.teichmann@intevation.de
bernhard.reiter@intevation.de = bernhard@intevation.de