Merge branch 'mr_shifted_periodicity' into 'master'

src/CMakeLists.txt

src/boundary/CMakeLists.txt

src/boundary/ShiftedPeriodicity.h

src/gpu/CMakeLists.txt

src/gpu/FieldAccessor.h

src/gpu/FieldAccessor3D.h

src/gpu/FieldAccessorXYZ.h

src/gpu/ShiftedPeriodicity.cu

src/gpu/ShiftedPeriodicity.h

tests/boundary/CMakeLists.txt

tests/boundary/TestShiftedPeriodicity.cpp

tests/boundary/TestShiftedPeriodicitySetup.py

tests/gpu/CMakeLists.txt

tests/gpu/TestShiftedPeriodicityGPU.cpp

tests/gpu/TestShiftedPeriodicitySetupGPU.py