Major updates:
- New 'fitnessDropoutPseudocount' option to add pseudocounts to output replicates with dropout i.e. variants present in input but absent from output (default:0). This is a 'workaround' for these (typically deleterious) variants whose fitness scores would otherwise be undefined or entirely missing from DiMSum output files. Note that pseudocounts are added during stage 5 and after error modeling to ensure that pseudocounts do not contribute to the error model fit.