BoundaryScrapingDiagnostic: periodically flush the buffers

[RemiLehe]

As a follow up of #3064, we would need to periodically flush the buffers that are used to store the scraped particles.
Right now, the BoundaryScrapingDiagnostic records the scraped particles throughout the simulation, and only dumps them to disk at the very end of the simulation. However, these buffers could end up using a large amount of memory.
Instead, we would need to flush them periodically to disk, similarly to what we do for boosted-frame diagnostics.