Add a pluggable linear solver of a vector equation

[yungyuc]

Augment the code for the Euler solver to allow changing the Jacobian matrix for any linear vector equations. It can be initially tested by using a $1\times1$ matrix for a scalar equation.

Linear vector equations can be tested with the stress waves in solids of cubic symmetry (or another symmetry) (see https://etd.ohiolink.edu/acprod/odb_etd/etd/r/1501/10?clear=10&p10_accession_num=osu1313000975)