Workflows and tests in separate scaling

[rantahar]

Not complete, but feel free to merge any time.

• Set cache-version to one explicitly to enable caching (probably not necessary)
• Fetch cmdstanr and iwmm from GitHub.
• Specify XML as a dependency to avoid problems with old version

[rantahar]

Tests are passing on my system and the workflows runs.

[n-kall]

Fantastic, thanks for all the work!