Replace error stop with s_mpi_abort in NaN timestep output check

CLAUDE.md forbids error stop; use s_mpi_abort for proper MPI-aware
termination. The print * before the abort is retained since it logs
the exact coordinates (j,k,l), variable index, rank, and timestep —
information that doesn't fit in the abort message string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_start_up.fpp

@@ -965,7 +965,7 @@ contains
                     do j = 0, m
                         if (ieee_is_nan(real(q_cons_ts(stor)%vf(i)%sf(j, k, l), kind=wp))) then
                             print *, "NaN(s) in timestep output.", j, k, l, i, proc_rank, t_step, m, n, p
-                            error stop "NaN(s) in timestep output."
+                            call s_mpi_abort("NaN(s) in timestep output.")
                         end if
                     end do
                 end do