Created function run_simul_shiny

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

It works well but it's slow because of the optim procedure : lam_Nt > calibrate vr > Leslie matrix as f(Nt)

changed name survival_mean > survivals
changed name fecundities_mean > fecundities

devtools : added package on Git