Flush target newstep

[franzpoeschel]

• merge Workaround for independent writes to Iterations in parallel, better detection of BP5 which in turn uncovers more instances of the first issue #1619 first
• documentation
• reading logic, e.g. adios2.engine.mode = "what's the name of the mode that we need here does it even exist yet help"
  It seems this needs no explicit reading logic, just io.SetParameters("FlattenSteps=on"); on the writer side?
• Automatically add that option in file-based encoding Not possible without breaking other workflows

Introduces a new flush target "new_step" used for creating readable output in file-based iteration encoding while the Iteration is not yet fully defined yet.

Close #1629

[ax3l]

This is awesome.

Let us also add a parallel test to cover serial and parallel I/O.

[franzpoeschel]

This is awesome.

Let us also add a parallel test to cover serial and parallel I/O.

Done as part of follow-up #1638