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.
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