Extend lists for simulation in genome mode to avoid IndexError

[lcoombe]

• In genome mode, it was possible to get IndexErrors like those reported in #228 if the lengths of two accessory lists happened to be shorter than the number of remaining reads to simulate
  • In the previous version of the code, multipliers were used to sample more data points from the distributions
  • In many cases, these were sufficient, but not in all cases, particularly when the number of remaining reads was smaller
• Refactored that logic into a separate method, and added a loop to increase the multipliers until both lists are longer than the number of remaining reads to avoid getting that error
• Added an iteration limit to avoid an edge case that lead to an infinite loop - I can't see how in practice this would happen, though