Supplement documentation to DirectLinearSolver

[bmxam]

This is a proposal to specify how to select the DirectLinearSolver, i.e : "use the MatrixRepresentation". This is an answer to #189 .

I don't know why, but I failed to compile the documentation locally, at least the settings.jl part. Before activating the github build action, maybe you can just take a look at the modifications and give me some feedback?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/settings.jl	100.00% <ø> (ø)

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[gdalle]

Thanks, this is nearly good to go!
I just added a remark on the kwargs because I have not yet fully stabilized the linear solver API (and may turn it into an abstract interface instead of always choosing a specific package).