Enable mixed usage of job nodes and job chains in nested job chains

Current Situation

• Nested job chains are limited to include a number of other job chains.

Desired Behavior

• Mixed usage of job nodes <job_chain_node> and included job chains <job_chain_node.job_chain> should be possible.

Maintainer Notes

• This feature requires a more thorough discussion:
  • either we will regard this feature when implementing Y job chains
  • or we will drop it should a decsion be taken to use an external workflow engine that might provide some similiar functionality