Added choice of model_demo inside the run_simul function using the set of decisions rules based on :
* lambda (growth vs decline) for DD
* lambda and N0 (small vs large pop) for StochDemo

Updated other functions accrodingly

1- Change ref pop size min = 0 by min = 30
Thus modified
  N_ref[N_ref == 0] <- NaN
   by
  N_ref[N_ref < 30] <- NaN

2- added warning message (if too many iterations with N_ref < 30)

3- updated description

Updated plot_impact accordingly

Added imports (magrittr, popbio) to models M3 and M4 functions