Supporting changes for the `assimp` rework in ITAGeo

ITAGeo was reworked to use assimp with the master thesis of Niklas Mika. Since both assimp and gRPC require zlib and shipped with it as a submodule, errors came up when building the ITASuite. These changes are needed to solve them. In addition, the urban simulator was adapted to support the changes.

Requires merge of ma23-mika in ITAGeo (see ITAGeo!3 (merged))!