Avoid sign mismatch in loop

[jlmelville]

GCC is complaining about different signedness in the loop which populates the return value in getDataByLabel:

warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
     795 |         for (int i = 0; i < dim; i++) {

The simplest fix, given here, is to change the type of the loop index i to size_t. A more idiomatic C++ approach would be to avoid introducing the loop in the first place, e.g.

std::copy(data_ptr, data_ptr + dim, std::back_inserter(data));

I checked that this worked (at least via the R bindings) but admittedly made no effort to do a proper performance check, so simpler is probably better.

[yurymalkov]

Thank you for the fix, @jlmelville!
I guess simpler is better due to readability (it looks more language agnostic). Though I am not sure why it is not just:

 for (size_t i = 0; i < dim; i++) {
      data.push_back(data_ptr[i]);
  }

Probably will change it to this.