Compile paths into absolute paths

[grst]

Description of feature

Since we now gitignore compiled params.yaml files I'm questioning the need for relative paths and stage_here. In principle it should be possible to compile !path xxx/yyy into absolute paths in every checkout.

it is a bit annoying that it's not backwards-compatible, though. I had hoped that stage_here doesn't touch absolute paths, but that doesn't seem to be the case. mabye we can address that in the dso-r R package though

[grst]

Depends on Boehringer-Ingelheim/dso-r#24