clock() measures total CPU time across all threads, not wall-clock
time. With the new OpenMP parallel regions in bssn_rhs_c.C, clock()
sums CPU time from all OpenMP threads, producing inflated timing that
scales with thread count rather than reflecting actual elapsed time.
MPI_Wtime() returns wall-clock seconds, giving accurate timing
regardless of the number of OpenMP threads running inside the
measured interval.
Co-authored-by: ianchb <i@4t.pw>