Add support for writing parallel structured VTK formats (.pvti / .pvtr / .pvts)

[corentin-dev]

Here is a PR to close issue #91

What was added : support for PRectilinearGrid, PStructuredGrid and PImageData.

WholeExtent needs to be improved. I am tempted to suggest using MPI here, but it would add a dependency? Any idea about this?

[corentin-dev]

@amartinhuertas maybe you have some ideas on how to improve this PR?

[jipolanco]

Thanks! I'm not yet sure about the best solution for the WholeExtent issue, but I think it's best to avoid depending on MPI.

I guess one simple solution would be to ask the user to explicitly pass extent and whole_extent as keyword arguments of pvtk_grid when writing structured grids. In parallel jobs, as a user one would like to have control of that anyway. Note that extent is already accepted by vtk_grid, but is not really documented...

[codecov-commenter]

Codecov Report

Merging #98 (17fe736) into master (a0344d2) will increase coverage by 0.43%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   95.58%   96.01%   +0.43%     
==========================================
  Files          16       16              
  Lines         770      803      +33     
==========================================
+ Hits          736      771      +35     
+ Misses         34       32       -2     

Impacted Files	Coverage Δ
src/gridtypes/common.jl	96.66% <100.00%> (+1.01%)	⬆️
src/gridtypes/pvtk_grid.jl	99.25% <100.00%> (+0.28%)	⬆️
src/gridtypes/structured/imagedata.jl	100.00% <100.00%> (+5.55%)	⬆️
src/gridtypes/structured/rectilinear.jl	100.00% <100.00%> (ø)
src/gridtypes/structured/structured.jl	86.27% <100.00%> (+0.27%)	⬆️
src/gridtypes/array.jl	85.71% <0.00%> (-1.79%)	⬇️
src/gridtypes/unstructured/polyhedron.jl	93.75% <0.00%> (-0.70%)	⬇️
src/gridtypes/unstructured/unstructured.jl	94.59% <0.00%> (-0.15%)	⬇️
src/write_data.jl	96.40% <0.00%> (-0.07%)	⬇️
src/gridtypes/ParaviewCollection.jl	97.72% <0.00%> (-0.06%)	⬇️
... and 2 more

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[jipolanco]

I think I got all structured formats working correctly.

For now the partitioning must be specified by passing an extents array, which contains the subdomains associated to each process. In MPI applications, each process must know what is the portion of the domain associated to all other processes. This may require some MPI communications before calling pvtk_grid.

I'm open to other ways of specifying the extents, but I'd prefer if we could avoid depending on MPI. If it's really worth it, we could try to support MPI via Requires.

I'll try to add some examples to the documentation. Besides, one annoying thing about parallel structured grids in VTK is that subdomains must overlap, or otherwise VTK (or at least ParaView) complains when loading the files. I'll mention this in the documentation.

[corentin-dev]

Good, I hope my work helped a bit ;-)

I'll try to use it in a parallel, though I don't have overlap, so we will see how it is handled...