[Jungfrau][Dark] Feat: Skip first ntrains

Description

This MR is addressing the request made by Marcin, https://git.xfel.eu/calibration/pycalibration/-/issues/122, to have a parameter in Jungfrau Darks to exclude the first number of trains from dark constants' processing. His analysis lead that the first trains of Jungfrau are a transient period that when we exclude we produce better dark constants

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

Reviewers

@mramilli @schmidtp @kluyvert