Issue in selfcal module when getting `spwmap`

[lucatelli]

An issue needs to be fixed for the following selfcal run:

• Only steps 'p0' and 'ap1' are executed
• If step 'p0' uses combine='spw' and step 'ap1' uses the bright template, the corresponding spwmap for ap1 will be wrong, as it will not use the spwmap from step p0.
  This is a typical run for e-MERLIN observations. Sources may be bright, but we should avoid performing multiple steps of selfcal, specially if the data has been flagged out significantly during phase referencing calibration.