Supress warning from flagByScatterLowpass

flagByScatterLowpass is currently emmitting the following warning:

/home/nils/Work/SaQC/saqccourse/venv/lib/python3.10/site-packages/numpy/lib/nanfunctions.py:1879: RuntimeWarning: Degrees of freedom <= 0 for slice.
  var = nanvar(a, axis=axis, dtype=dtype, out=out, ddof=ddof,

which is unfortunate and results in a lot of questions during the courses and trainings. As everything works as expected, I guess we should suppress this warning...