Merge branch 'xtensor-refactor-2' into 'master'

idg-api/Buffer.cpp

idg-api/BufferImpl.h

idg-api/BufferSet.cpp

idg-api/BufferSetImpl.h

idg-api/BulkDegridder.cpp

idg-api/BulkDegridderImpl.h

idg-api/DegridderBuffer.cpp

idg-api/DegridderBufferImpl.h

idg-api/GridderBuffer.cpp

idg-api/GridderBufferImpl.h

idg-bin/tests/cxx/common/common.cpp

idg-lib/src/CPU/common/CPU.cpp

idg-lib/src/CPU/common/CPU.h

idg-lib/src/CUDA/common/CUDA.h

idg-lib/src/CUDA/common/routines/Beam.cpp

idg-lib/src/common/Proxy.cpp

idg-lib/src/common/Proxy.h