AST-1292 Adapt tPhaseShift test process methods to unique_ptr interface

Malcolm MacLeodrequested to merge
ast-1292-tphaseshift-use-xtensor into master

Adapt PhaseShift unit tests in tPhaseShift to use new DPBuffer interface and XTensor:

  • Update all process() interfaces and calls to use unique_ptr
  • Replace all Casacore classes/variables/functions with XTensor equivalents. With the exception of antenna positions/phases, these can't yet be updated until the antenna classes and DPInfo provide a new interface.

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