Unique varlist and choosing endogenous variables for estimation

- Make sure that only unique entries of var_list_ are processed. Prevents redundant results and unnecessary crashes.

- Adds new option for selection of endogenous variables for estimation (#336)

TBD: 
* add two tokens like ```consider_all_endogenous``` and ```consider_only_observed``` to ```estimation``` that set ```options_.endo_var_estimation``` to ```all_endogenous_variables``` and ```only_observed_variables```, respectively
+ add those to the documentation