Add parallel ILU0

[jkrch]

I added a parallel version of ILU0, based on this paper.
To get more parallelism out of the problem the linear system gets reordered, based on a coloring of the graph of the matrix and then the reordered system is solved instead of the original problem.
The workflow looks something like this:

P = ParallelILU0Preconditioner(A)
A_reord, b_reord = reorderlinsys(A, b, P.coloring)
sol = gmres(A_reord, b_reord, Pl=P)

[codecov-commenter]

Codecov Report

Merging #13 (a604f25) into master (d8551f2) will increase coverage by 2.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   87.07%   89.38%   +2.31%     
==========================================
  Files          15       16       +1     
  Lines         588      688     +100     
==========================================
+ Hits          512      615     +103     
+ Misses         76       73       -3     

Impacted Files	Coverage Δ
src/ExtendableSparse.jl	100.00% <ø> (ø)
src/factorizations.jl	73.33% <ø> (+6.66%)	⬆️
src/parallel_ilu0.jl	100.00% <100.00%> (ø)
src/ilu0.jl	100.00% <0.00%> (+3.70%)	⬆️

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