Revive Spark integration

[untitaker]

the spark integration is currently unmaintained and we don't have the resources to fix the testsuite. The last pieces of it have been removed from CI in #1101

[pvanderlinden]

It would be good to update the documentation for now I would say. The integration currently actually causes a lot of exception, see the above Issue.

[sentrivana]

Will close this since we revived the test suite in bcc563c

We'll deal with the specific problems with the integration in the individual issues.