individual readouts to bunches and align them instead. This way
one can overcome poor statistics.

- hot pixels 0 after dark subtraction, if masking is used
- hot pixels dark mean if masking is used but dark subtraction is not

Centroid function modified to always output spectra

centroiding is not always fine

this might be wrong and thus confusing

do not store the data in our own class, employ xarray for that.