Block assemblers for v0.3

[JordiManyer]

This PR implements block assemblers, which allow to assemble a MultiFieldFESpace block-wise. It is the distributed counterpart of https://github.com/gridap/Gridap.jl/pull/915.

Instead of a sparse monolithic matrix and vectors, this assembler leverages BlockArrays.jl to produce equivalent block-partitioned sparse matrices and vectors. This is meant to ease the development of block preconditioners and solvers.

TODO:

• [x] Find a reliable way to compare solutions between block-assembled and monolothic systems.
• [x] Right now we have a problem dealing with different blocks having different PRanges. This complicates the allocation of a single column vector for the whole matrix, since ghosts do not match when multiplying with different blocks in the same block column. We will have to homogenize the row/col ghost values for all blocks in the same row/column by comparing them and keeping the biggest set.
• [x] Should we move the block linear algebra operations here from GridapSolvers?
• [x] Should we move to_parray_of_arrays and local_views implementations for BlockArrays to the Algebra.jl file?
• [x] To solve problem 2. here, should we try to implement a change_ghosts variant for Block-Multifield, like in the tests? This would give us a way to move from dof partitions to matrix partitions (since having identical PRanges is for now discarded as a solution). Or we could just not solve the issue, since it is already present for non-block assembly as well.

Other pains/doubts about the implementation can be found in https://github.com/gridap/GridapSolvers.jl/issues/29.

[codecov-commenter]

Codecov Report

Merging #124 (90df49b) into master (234646e) will not change coverage. The diff coverage is 0.00%.

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@@           Coverage Diff           @@
##           master    #124    +/-   ##
=======================================
  Coverage    0.00%   0.00%            
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  Files          11      12     +1     
  Lines        2082    2448   +366     
=======================================
- Misses       2082    2448   +366     

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

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[JordiManyer]

Hey @amartinhuertas , I think I'm ready to merge this. Any final thougts?

[amartinhuertas]

Hey @amartinhuertas , I think I'm ready to merge this. Any final thougts?

No. Please go ahead.