Add estimated time remaining (ETA) to simulation time-step output

[sbryngelson]

Summary

The simulation already prints progress info every t_step_print steps, including Time Avg (running average wall time per step) and Time/step (current step wall time). We should use this data to compute and display the estimated time remaining (ETA).

Current output

Fixed timestep mode:

 [ 25%]  Time step 250 of 1000 @ t_step = 250  Time Avg = 1.234567E-01  Time/step= 1.200000E-01

CFL-based dt mode:

 [ 25%] Time  1.250000E-03 dt =  5.000000E-06 @ Time Step =      250  Time Avg =  1.234567E-01  Time/step = 1.200000E-01

Proposed change

Append an ETA field to each line computed from:

ETA = wall_time_avg * steps_remaining

where steps_remaining = t_step_stop - t_step (fixed dt) or estimated from (t_stop - mytime) / dt (CFL mode).

Format the ETA in human-readable form (e.g., 2h 13m 05s or 45m 30s or 12s).

Example output:

 [ 25%]  Time step 250 of 1000 @ t_step = 250  Time Avg = 1.234567E-01  Time/step= 1.200000E-01  ETA = 1h 33m 08s

Implementation

All changes are in src/simulation/m_start_up.fpp, lines ~821-839 (s_perform_time_step):

1. Compute steps_remaining:
   • Fixed dt: t_step_stop - t_step
   • CFL dt: nint((t_stop - mytime) / dt) (approximate, since dt can change)
2. Compute eta_seconds = wall_time_avg * steps_remaining
3. Convert to hours/minutes/seconds integers
4. Append to the existing print format string

This is a ~15-line change in a single subroutine. No new parameters needed — it uses existing wall_time_avg, t_step_stop, t_step, t_stop, mytime, and dt.