[Core] Add Process for Flushing Streams

[matekelemen]

Nearly every MainKratos.py I've ever seen begins by defining an AnalysisStage that flushes stdout regularly. In this PR, I'm adding a process that does the same thing to make the main python scripts cleaner.

[matekelemen]

I would make this part of the analysis stage, see what I did in the CoSim Analysis Stage. I quite like the solution, it is simple yet worked well for me

fine by me, it's a matter of whether the @KratosMultiphysics/technical-committee is ok with touching AnalysisStage.

[rubenzorrilla]

I'd follow @philbucher 's suggestion as it keeps the "automatic" handling of the flush, which I find is more convenient for the user.