Integration tests

[gmertes]

With core now having nightly integration tests, we want to add a similar setup for inference.

An initial setup could run from a prepared grib file as input and output to grib. Then do some sanity checks on the output grib (statistics, key names, etc).

[frazane]

Looking forward to use the new setup! Just one comment: I would make sure to also add some developer documentation on how the integration tests setup works, and how add new tests.