Add the length of the squiggle to the debug log

[Adoni5]

Currently we only write out the lenght of the sequence, but for debug purposes it would be useful to know what length of signal produced said sequence.
Would need to add a header here:

• readfish/src/readfish/_statistics.py

  Line 37 in c6e52c7

  DEBUG_STATS_LOG_FIELDS = (

And pass it into the logread call here:

• readfish/src/readfish/entry_points/targets.py

  Line 517 in c6e52c7

  self.loop_statistics.log_read(