Fix Adams-Bashforth checkpointing

[ia267]

Summary

• Extend ADIOS2 checkpoint to support Adams-Bashforth time integrators:

  • detect AB time steppers, preserve istep/nstep and write every olds(i,j) buffer per rank in its native SIMD layout
  • keep those buffers alive until writer_session%close() (ADIOS2 deferred writes) and restore them without reordering
  • zero velocity and history fields before restore so padding is deterministic

• Add a lightweight test case for checkpoint testing with AB3 time-integrator (runs a tiny 1x1 toy mesh, with deterministic derivatives, writes a checkpoint at iteration 5, restarts immediately and checks every old(i,j) buffer matches bit-for-bit)

Testing

• continuous vs. restart runs for the TGV case works as expected for AB1-4.
• Ran an AB3 case from 0 to 100 with a checkpoint at 50, restarted from 50 to get bit identical results to the continuous run at time-step 100 (compared enstrophy at this time-step)

closes #174