Optional support for parallel HDF5 I/O

[jipolanco]

This adds support for writing and reading MPI-distributed fields (PencilArrays) using parallel HDF5.

The HDF5.jl package is loaded using Requires, and therefore it's not added as a dependency of PencilFFTs.

Things to do:

• [x] Write fields to HDF5 group/file
• [x] Try to keep the API compatible with HDF5.jl: define setindex! and others in a compatible way
• [x] Wrap h5open for parallel I/O (it's quite messy otherwise)
• [x] Read fields (in-place, ideally)
• [x] Write/enable HDF5 tests
• [x] Defaults
  • [x] Write chunked (ideally 1 chunk per process) or contiguous data?
  • [x] Collective operations should be default.
• [x] Docs
  • [x] Add tutorial or example
  • [x] Setting up HDF5 with parallel support
  • [x] Order of imports (PencilFFTs last)

[codecov[bot]]

Codecov Report

Merging #11 into master will increase coverage by 0.79%. The diff coverage is 93.97%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage   86.20%   87.00%   +0.79%     
==========================================
  Files          20       22       +2     
  Lines         725      808      +83     
==========================================
+ Hits          625      703      +78     
- Misses        100      105       +5     

Impacted Files	Coverage Δ
src/PencilFFTs.jl	100.00% <ø> (ø)
src/PencilArrays/arrays.jl	83.33% <50.00%> (-0.82%)	:arrow_down:
src/PencilIO/hdf5.jl	94.93% <94.93%> (ø)
src/PencilIO/PencilIO.jl	100.00% <100.00%> (ø)

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