Add SelectNthEventsProcessor

[ueinhaus]

BEGINRELEASENOTES

• Adds SelectNthEventsProcessor.
• This sets its own processor ReturnValue to true or false, depending on chosen parameters, which can be used in the Marlin steering file to use a particular sub-set of a sample.
• Via the InvertSelection parameter this can be easily inverted, which is convenient for training vs. inference of ML models.
  ENDRELEASENOTES

[tmadlener]

Will such a regular selection of events potentially run into any statistical issues? E.g. compared to a random sampling?

[ueinhaus]

I would rather make a regular selection than make sure that a random selection is reliably repeatable. In addition, the event samples are intrinsically randomised, so it should not have an effect on the physics.