Adds the CppTest marker-interface to exclude Healer tests that  were
interfacing with CGAL from the build. These tests are needed for the
development of the replacement library, but shall not cause build
failures in the meantime.

BREAKING CHANGE: Implements the saving and loading of CityObjects to a
GeoDB with in-memory caching. This allows reading of bigger CityGml
files and serves as basis for the implementation of Topological Checks.

CityDoctorModel will now return Streams instead of Lists for the
CityObject-getters (like getBuildings()). This requires a rework of all
methods calling them.

(cherry picked from commit 2bf3cfb6)