gadget_iterative_stage_* mobilizes only 1 core

I can't explain why monitoring the system while running the procedure below shows that only 1 core is up running.

```
gadget_iterative_stage_1(gd, params.in = 'params.in') %>% 
       parallel::mclapply(gadget_optimize, mc.cores = parallel::detectCores()) %>% 
       gadget_iterative_stage_2() %>% 
       gadget_optimize()
```
