Add support for IDGCal

This MR adds support for IDGCal (see RAP-762). To save compute time, the use of IDGCal is limited to the final processing cycle (i.e., the normal solve is used during selfcal). Once we understand better how IDGCal performs (e.g., with GPUs, with selfcal convergence, etc.), we can consider using it during selfcal as well.