CI: caching of dependencies

[volkm]

CI runs of Storm/py/wheels/.. have seen errors in the last weeks due to unavailable 3rdparty dependencies, most notably gmm and Eigen. This is mainly due to high load or rate limiting on the corresponding servers. For example, Gitlab, which hosts Eigen, will introduce rate limiting on Oct 19th.

I would propose to make the CI more robust through either of:

1. improve the caching such that dependencies do not need to be re-downloaded. I am not sure whether this is possible with PR caches which are typically isolated per PR.
2. already include the dependencies in the base images and do nightly/weekly rebuilds of all base images.
3. ship again with the sources of all dependencies. (But for maintenance, this is probably not a good option.)

[sjunges]

Yes, nr 2 sounds like something we may need to do. We should have weekly builds that also not install these things and see whether our cmake is correct then.