Fix issues with transpositions on GPUs

[jipolanco]

Fixes the following issues when transposing distributed CuArrays:

• sometimes the wrong permutation was applied to the output of a transpose
• MPI buffers cannot be created directly out of CuPtrs
• gather sometimes gave wrong results on distributed CuArrays

[codecov-commenter]

Codecov Report

Merging #65 (072554e) into master (f89299b) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   97.01%   96.97%   -0.04%     
==========================================
  Files          22       22              
  Lines        1208     1192      -16     
==========================================
- Hits         1172     1156      -16     
  Misses         36       36              

Impacted Files	Coverage Δ
src/Transpositions/Transpositions.jl	98.06% <100.00%> (-0.03%)	:arrow_down:
src/PencilIO/hdf5.jl	93.33% <0.00%> (-0.11%)	:arrow_down:
src/PencilIO/mpi_io.jl	96.12% <0.00%> (-0.08%)	:arrow_down:
src/Pencils/MPITopologies.jl	97.40% <0.00%> (-0.07%)	:arrow_down:
src/Pencils/Pencils.jl	99.13% <0.00%> (-0.02%)	:arrow_down:
src/gather.jl	100.00% <0.00%> (ø)
src/Pencils/data_ranges.jl	100.00% <0.00%> (ø)

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