set delay calibration as default in selfcal

[Fil8]

better practice is to apply delay self-calibration with cubical in the self-cal loop. We need to change the default parameters to Fslope with sensible numbers. For an 8s dumptime of visibilities a good number could be gsols_timeslots:2 which is similar to 16s.

[paoloserra]

@Athanaseus @francescaLoi @Fil8 should we make this change in the schema (which has the disadvantage that people updating Caracal will get a different result with the same config file if they have not explicitly set the changed parameters), or in the sample configs at https://github.com/caracal-pipeline/caracal/tree/master/caracal/sample_configurations ? The problem with the latter is that I don't know how widely they are used...