AST-1292 Adapt PhaseShift process method to new interface and XTensor

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

Adapt PhaseShift to use new DPBuffer interface and XTensor:

  • Update all process() interfaces and calls to use unique_ptr
  • Replace most Casacore classes/variables/functions with XTensor equivalents. With some exceptions that can't yet be updated.

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