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.

#-*-makefile-*-
# Authors: Sascha Wilde <wilde@intevation.de>

.PHONY: all dependencies clean

all:	dependencies
	yarn build

dependencies:
	yarn install --check-files

clean:
	rm -rf ../web