Validation for Unset Linear Solver #598
Description
When using the Newton solver with the linear solver unspecified, a segfault can occur at
https://github.com/LLNL/sundials/blob/c28eaa3764a03705d61decb6025b409360e9d53f/src/sunnonlinsol/newton/sunnonlinsol_newton.c#L247
This was noticed in #597 which uses CVDOE, and I've seen similar behavior with ARKODE.
Additional validation is needed to alert users that a linear solver should be specified.