Validation for Unset Linear Solver

[Steven-Roberts]

When using the Newton solver with the linear solver unspecified, a segfault can occur at

sundials/src/sunnonlinsol/newton/sunnonlinsol_newton.c

Line 247 in c28eaa3

retval = NEWTON_CONTENT(NLS)->LSolve(delta, mem);

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.