Implement minimal application code for testing

[oleschwen]

We should have a minimal, fast-running application code for integration testing. Should finish training in seconds.
Check if the NVFlare examples contain something suitable for this purpose and if it can be used with the MediSwarm swarm learning.

[oleschwen]

application/test_jobs/minimal_training/ (at 19c4b85) takes about 30 seconds in simulation mode. Most of this is setup of the simulation, the actual training takes about 4 seconds.

[oleschwen]

The base model and data module are the same as in the 3D CNN example and could be extracted as application code infrastructure. I'll create a follow-up issue for this.