Change Docker images used in CI
We use prebuilt Docker images on the `ajaust` namespace and push them to DockerHub. This commit makes sure that the CI is using these new container images. These changes have been done while moving the CI to a new VM. Related changes: * Tolerance of partitioned tests was increased to `5e-5` as the results changed slightly using the new Docker images on the new VM. * Added a copy of `fuzzycomparevtu.py` from the DuMuX repository into this repository. This allows us to be more independent of the location of the DuMuX repository.