`to_xp`: AoS, SoA, PODVector, MF, Array4

[ax3l]

Add a to_xp() method that returns either NumPy or CuPy containers, depending on amr.Config.have_gpu, to all major AMReX data containers.

Close #216

To Do

• [x] implement
• [x] update tests, disable default managed in tests
• [x] advertise prominently in zero-copy APIs and compute examples
• [x] run all tests on GPU (CUDA)