Support Adams-Bashforth time integration in checkpoint

[ia267]

Current ADIOS2 checkpoint implementation only properly handles Runge-Kutta time integration methods. Adams-Bashforth methods (AB2-AB4) requires saving additional state information (previous derivatives) to properly restart simulations.

Implementation details:

• Extend checkpoint_io.f90 to identify when Adams-Bashforth methods are used
• Add support for storing previous derivatives (or flow field) in a olds array
• Preserve the time stepper counter (iter) during checkpoint/restart
• Handle proper AB startup sequence after restarts

Acceptance criteria:

• Simulation can be properly checkpointed and restarted with AB2, AB3 and AB4 time integration methods
• Numerical results before and after are identical (within floating point tolerance)
• No performance degradation for non-AB methods