Add interface to set LaTeX-name of optimal_policy_discount_factor

I would propose an option planner_discount_latex_name to ramsey_model that takes a string and sets M_.param_names_tex{strmatch('optimal_policy_discount_factor',M_.param_names,'exact')} as well as the internal field in the preprocessor output used for the write_latex_* commands. Unfortunately, this needs to be a LaTeX string that can contain a backslash. If the parser cannot easily handle this, we should not implement it.