Transition from using Casacore arrays to XTensor in DPBuffer

The transition from the use of Casecore arrays to XTensor in DP3 will

• improve maintainability of the code by avoiding Casacore reference semantics;
• improve scalability by using the thread safe XTensor library;
• improve readability of the code (and hence improve its maintainability and reduce development time);
• enable use of various code optimisations from the XTensor library or libraries depending on XTensor for computational performance improvements;
• Reduce the dependency of DP3 on Casacore.

• The first three steps in the transition path described on https://confluence.skatelescope.org/display/SE/Transition+to+XTensor have been implemented in all DPSteps, so that XTensor can be used but DP3 still supports use of Casacore arrays as well (and thus remains backward compatible).
• A follow-up feature has been defined to start work on steps 4 through 7 on the transition path.

Due to an unforeseen capacity reduction, not all DPSteps are using unique pointers and XTensor views yet. The linked stories give a nice overview of the work done. A few stories are still in progress and are likely going to be completed before PI planning. The remaining stories will roll over to PI19, for which a follow-up feature (SP-3413) was created. This follow-up ticket also includes the actual removal of Casacore arrays from the DPBuffer, which will then work with XTensors instead.