make it resistant to potential const correctness fix in Rcpp. 

This is related to RcppCore/Rcpp#211

The fix is minimal and just makes sure the code works with both released Rcpp and a version of Rcpp that is more correct regarding const ness of Vector iterators. 

I guess a better fix would be to use this signature in `c_quantile`: 

```
double c_quantile(NumericVector::const_iterator data, const int n, const double quantile, const bool sorted=false)
```

since you are sending it an iterator to the beginning of a const vector. However you are then using `nth_element` on the pointer, so you won't respect constness. Then not sure why you use `const NumericVector` in the first place in this: 

```
RcppExport SEXP c_quantile2(SEXP data_, SEXP quantile_) {
  const NumericVector q(quantile_);
  const NumericVector data(data_);
  const int n = data.size();
  const int nq = q.size();
  NumericVector res(nq);
  
  for(int i = 0; i < nq; ++i)
    res[i] = c_quantile(const_cast<NumericVector&>(data).begin(), n, q[i]);
  
  return res;
}
```

The call `res[i] = c_quantile(data.begin(), n, q[i]);` works with current Rcpp because it does not respect const ness and gives you a non const pointer where it should really give you a const one.