nullables in Cpp functions

[diazrenata]

Warning messages:
1: Unable to parse C++ default value 'NULL' for argument local of function dataFunCpp

https://gallery.rcpp.org/articles/optional-null-function-arguments/

int intFunCpp(Rcpp::StringVector fun_name,
                Nullable<NumericVector> probs=R_NilValue, 
                Nullable<int> x=R_NilValue) 

[ajrominger]

related to #112