Made Model 3 (with DD) simulations faster by changing the calibrate_params function (based on optim) by an analytical approximation (using match_lam_delta function).

Tests reveals that the approx is largely good enough. Makes no differences in the results.

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

updated variables names in the run_analysis file

changed name survival_mean > survivals
changed name fecundities_mean > fecundities

devtools : added package on Git