Bug for asynchronous specs of skip_step and save_every in ExportCallBack

[mstoelzle]

Describe the bug
If save_every is not a multiple of step_skip, the condition (current_step + 1) % self.save_every == 0 (https://github.com/GazzolaLab/PyElastica/blob/master/elastica/callback_functions.py#L205) will never be trigged and the data will never be saved, if the condition about the buffer size does not become active neither.

To Reproduce
This code snippet below has a time-step of 0.1ms. It should save the data to the buffer every 20ms and it should dump the buffer to the file every 1.0001ms.

dt = 1e-4
step_skip = 2e2
save_every = 10001
callback = ExportCallBack(step_skip, ".", "tempfile", save_every=10)
simulator.collect_diagnostics(stretchable_rod).using(
     callback, 
     step_skip=step_skip, 
     path=".", method="tempfile", 
     save_every=save_every,
)

However, in this case the (current_step + 1) % self.save_every == 0 will never be triggered, as it never reaches there as this time-step is skipped by step_skip=2e2. PR #125 would fix this.

Environment

• Python=3.10.5, pip=22.1.2
• Numpy/numba version: 22.1.2
• PyElastica version: 0.2.3
• OS, Device: macOS / macBook Pro 15'' 2019

Expected behavior
By specifying save_every, the data should be saved every save_every time-steps, irrespective of the chosen skip_step setting.

Screenshots
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Additional context
Add any other context about the problem here.

[skim0119]

Resolved with #128.