Exploring ghost optimisation

[JordiManyer]

Todo/Done:

• [x] Merge functionalities of consistent_local_views and change_ghost. Expand functionality of change_ghost.
• [x] Add possibility to use OwnAndGhostVector as vector partition of resulting PVectors, as a way to reduce memory copying when transferring between FESpace and linear system layouts.
• [x] Implement BlockPArray <: AbstractBlockArray, a new type that behaves as a BlockArray{PArray} and which fulfills the APIs of both PArray and AbstractBlockArray. This will get rid of the type piracy we have been using up to now and gets rid of some of the issues we are having (mainly, the fact that BlockArray expects its blocks to be indexable).
• [x] MultiFieldFESpace{<:BlockMultiFieldStyle} now has a BlockPRange as gids, which makes the fespace dof vector be of type BlockPVector. This is necessary to create consistency between fespace and system vectors, which in turn avoids memory allocations/copies when transferring between FESpace and linear system layouts.

To Think:

• Now BlockPArray constructors work with BlockPRanges, which is inconsistent with PartitionedArrays. Should we be trying to be coherent? How can we do it well for dispatching?

Left for another PR:

• Implement a new mechanism that allows the user to skip PRange ghost-related optimisations while assembling the system matrix. This will create linear systems which ghost layout is compatible with the original FESpace, avoiding copying data when transferring between FESpace and linear system layouts.

[codecov-commenter]

Codecov Report

Merging #128 (f73f0d6) into block-assemblers-v0.3 (25ece1f) will not change coverage. The diff coverage is 0.00%.

:exclamation: Current head f73f0d6 differs from pull request most recent head c8c6d2a. Consider uploading reports for the commit c8c6d2a to get more accurate results

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@@                  Coverage Diff                   @@
##           block-assemblers-v0.3    #128    +/-   ##
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  Coverage                   0.00%   0.00%            
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  Files                         11      12     +1     
  Lines                       2242    2402   +160     
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- Misses                      2242    2402   +160     

Files Changed	Coverage Δ
src/Algebra.jl	0.00% <0.00%> (ø)
src/BlockPartitionedArrays.jl	0.00% <0.00%> (ø)
src/FESpaces.jl	0.00% <0.00%> (ø)
src/MultiField.jl	0.00% <0.00%> (ø)

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[JordiManyer]

Tests will not fail after the release of Gridap v0.17.20