Allow newtonSolveLinear to change the matrix, since this is required by some...

Allow newtonSolveLinear to change the matrix, since this is required by some solvers, e.g. AMG, and we do not want to make the extra effort of copying everything. In general, changing the matrix could have negative consequences for partial reassembly and Jacobian recycling. For AMG, this does not seem to be the case, partial reassembly and recycling work as before. Reviewed by Christoph.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9180 2fb0f335-1f38-0410-981e-8018bf24f1b0