Add Time-Averaged Field Diagnostics

[n01r]

This PR adds time-averaged field diagnostics to the WarpX output

• code
• docs
• tests
• example
• meta-data $\Longrightarrow$ Follow-up PR
• make compatible with adaptive time stepping $\Longrightarrow$ Follow-up PR

Currently, compiling for GPU creates a linking error. We would like to see the CI output to possibly find the issue.
Hence, we do not flag this as WIP right now.

This PR is based on work performed during the 2024 WarpX Refactoring Hackathon and was created together with @RevathiJambunathan. 🙂

Successfully merging this pull request may close #5165.

[archermarx]

Thanks for making this PR! One complication is that with #5176, adaptive timestepping will be possible. This means that the timestep length can change, so simply summing the multifabs and dividing by the period won't be enough for a proper time average. I don't think addressing this will be too hard, however.

[n01r]

Thanks, @archermarx! I wasn't aware of that yet. 🙂 We can do a follow-up and catch if dt_update_interval is set. In that case, we could create a cumulative sum weighted with the time step length and also add up the total time as we go. During the flush, we then divide by the total time.

[archermarx]

Thanks, @archermarx! I wasn't aware of that yet. 🙂 We can do a follow-up and catch if dt_update_interval is set. In that case, we could create a cumulative sum weighted with the time step length and also add up the total time as we go. During the flush, we then divide by the total time.

Yep, my thought exactly. The adaptive timestepping PR was just merged, too.

[EZoni]

I think #5296 should fix the clang-tidy errors that remain, which don't seem related to this PR.