Cone-beam support in pipeline

About

This MR adds integration of cone-beam reconstruction in the pipeline.

For the record, cone-beam reconstruction was already available from the python API, even by volume slabs.
Now the objective is to make it available from CLI/pipeline.

To do

• Rework ConebeamReconstructor interface to add sample-detector distance
• Adapt unit tests
• CLI interface
  • Config file: reconstruction method, geometry
  • Validator
• Compute the required overlap between slabs
• Adapt estimate_required_memory (Astra duplicates the projection data)
• Adapt nabu.pipeline.fullfield
• End-to-end tests