Double linear solve setups for Levenberg–Marquardt algorithm

[ChrisRackauckas]

There are two separate linear solves, so it would be good to allow linsolve and precs for the two different ones, defaulting to using the same choice for the two. I don't know how helpful this would end up really being in practice, though for iLU you may want to cache different sparsities or something?

[avik-pal]

Is this needed though? The linear solves are $A x_1 = b_1$ and $A x_2 = b_2$. The original version using 2 distinct $A$s was an incorrect implementation that has been removed.