Allow `adios2_inquire_group_variables` to list variables in the root group

[eschnett]

The function adios2_inquire_group_variables can list variables in subgroups, but not in the root group.

This might not be intentional; this could be a side-effect of the lines

                auto pos = curr.rfind("/");
                if (pos <= prefix_size)
                {
                    names.insert(curr);
                }

which check whether the found object is a leaf or a branch. Extending the if condition to pos <= prefix_size || pos == std::string::npos would (I think) allow using the empty string "" to query the root group.