Bugfix: add_entry! for PSparseMatrices

[JordiManyer]

Copied from GridapDistributed issue 138.

In Gridap, we declare the function Algebra.add_entry, which is used to assemble an (i,j,v) triplet into different structures (counters, COOs and matrices). As usual, there are different specializations of this function. In particular, we have different functions for AbstractMatrix and AbstractSparseMatrix:

function add_entry!(combine::Function,A::AbstractSparseMatrix,v::Number,i,j)
  k = nz_index(A,i,j)
  nz = nonzeros(A)
  Aij = nz[k]
  nz[k] = combine(v,Aij)
  A
end

@inline function add_entry!(combine::Function,A::AbstractMatrix,v,i,j)
  aij = A[i,j]
  A[i,j] = combine(aij,v)
  A
end

As we can see, for AbstractMatrix we call combine(aij,v) whereas for AbstractSparseMatrix we call combine(v,aij). This leads to different behaviors when combine is not commutative. For instance, (a,b) -> a would create different matrices depending on the type.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4a76c17) 88.14% compared to head (a639b11) 88.14%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #959   +/-   ##
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  Coverage   88.14%   88.14%           
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  Files         177      177           
  Lines       21064    21064           
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  Hits        18566    18566           
  Misses       2498     2498           

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