Improved notebook with reviewer suggestions.

 - added HDF5 import: `ont_fast5_api`
 - added results image dir creation
 - added seed for random read subset selection (reproducibility)
 - intermediate result caching is now done per-virus
 - removed multi-threshold code (unused)
 - removed hard-coded optimal thresholds, now recalculated and plotted
   using lambda phage data. Minor model adjustments to do so.

Author: TimD1