!Feature: Implement GeoDB and FeatureCache

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.