Generate an initial sky model from the input data

This MR adds the option to generate an initial calibration sky model from the input data (see RAP-212). A new ImageInitial operation class was added for this purpose that runs WSClean to image the full field, with the resulting clean components being used as the initial sky model. This imaging step is very similar to that done by LINC during the target selfcal (it is not a duplicate, however, because the image made by Rapthor includes the improved calibration corrections that result from the LINC selfcal).

Generating a model is now the default way of making an initial model when one is not supplied (so download is now off by default), as the quality of the generated model was found to be higher than that obtainable from a catalog. Due to the high quality of the generated model, the default strategy was adjusted so that the phase-only calibration cycles of selfcal are skipped when generation is activated.