Center peak regions at summit in `modisco seqlet-bed`

[austintwang]

Chrombpnet defines the peak center as start + summit, not the midpoint between start and end. It would be good to make write_bed_from_h5 consistent with this convention.

https://github.com/jmschrei/tfmodisco-lite/blob/d6716a56c407040e73f48cf0662a432035e4fde2/modiscolite/io.py#L349C1-L350C1

So absolute_peak_center = int(peak_row[1]) + int(peak_row[9]) instead