[eigen] ILU decomposition

[Peter Román <peterrom@xxxxxxxxx>]

Hi!

Thanks for a great lib :)

I made a patch to add (very basic) support for SuperLU's incomplete
factorization routine. I'm not sure if it's interesting for inclusion
or not, but here it is.

I also looked into adding additional inputs for ILU specific option to
the SparseLU class, but it seems no two available ILU packages are the
same, so it gets messy.

I would suggest adding a flag for whether to use the modified ILU (MILU) or not.

A (very minor) annoyance I have is that the default ordering method is
set as the default value of the flag parameter, so

SparseLU<SparseMatrix<double>, SuperLU> ILU(A, IncompleteFactorization);

leaves the ordering method undefined and thus generates an error message.

Thanks again,
Peter Román

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