Reconsider how to store pixel-level interval/sparsedata

[psomhorst]

PixelBreaths.find_pixel_breaths(...) returns an IntervalData object with a list of numpy arrays. this works, but is not ideal. It is not possible to slice the list to select specific pixels.

We should think about how to store pixel-level values in IntervalData or SparseData objects. This could be as arrays instead of lists (probably requiring adaptations for slicing) or in separate objects.