Update SegmentList usage

[jotaylor]

Instead of explicitly calling out STIS and COS, you can use the generic SegmentList since no data is actually being coadded together here.
https://github.com/spacetelescope/ullyses/blob/main/ullyses/timeseries.py#L192

[jotaylor]

I believe the original intention here was to abstract away the instrument-specific usage of child SegmentList classes in timeseries. This may be possible, but would require testing. This ticket will be left open for now, but this is an enhancement to generalize the code, not a necessity.