Bugfix: simulation_type_knockdown with multiple genes

[LouiseDck]

According to the vignette, this is the way to create a knockdown simulation:
0

b3_genes <- model_common$feature_info %>% filter(module_id == "B3") %>% pull(feature_id)

model_ko <- model_common
model_ko$simulation_params$experiment_params <- simulation_type_knockdown(
  num_simulations = 100L,
  timepoint = 0, 
  genes = b3_genes,
  num_genes = length(b3_genes),
  multiplier = 0
)

However, when there are multiple genes associated with the module, this did not work and produced relating to incompatible sizes in tibble().
This is due to the checks performed in simulation_type_knockdown.

Fixes #56
Fixes #45