Remove default reference antenna from v4 data sets
Remove default reference antenna from v4 data sets

The katdal "reference antenna" (no relation to the calibration ref ant)
is used for three things during construction of a DataSet:

  - It provides the activity sensor that partitions data set into scans
  - It provides the target sensor that identifies all the targets
  - It serves as the default antenna for the DataSet catalogue

The default reference antenna (obtained by setting ref_ant parameter
to '') is currently the first antenna in the list. This is very
problematic if the first antenna misbehaved (see e.g. tickets SR-1383
and COMM-342). In the absence of an explicit reference antenna (the
default case), now do the following:

  - Use the obs_activity sensor to partition the data set (this is what
    the array intends to do, not what individual antennas actually do)
  - Use the cbf_target sensor for targets (this is the delay tracking
    centre, a better array target than individual pointing centres)
  - Use the array reference part of the antenna object as a new default
    catalogue antenna (this is nominally the array centre as antenna)

Picking an explicit ref_ant still recovers the old katdal behaviour.

This addresses JIRA ticket SR-1188.

ska-sa/katdal