flush_target that ends/starts step

[ax3l]

For boosted frame simulations, we plan to use a one-file-per-lab-frame mode with many adios steps in each.
During read, we flatten the adios steps into one [1].

In openPMD-api for writes, we need to add a third option for flush_target that calls EndStep()/BeginStep() instead of PerformDataWrites().

ADIOS2 features:

• [1] ReadModeFlatten ornladios/ADIOS2#4147 (comment) -> Remove special ReadFlattenSteps Mode, switch to control by writer-sid… ornladios/ADIOS2#4160