reserve_capacity calculate error in sample_core.cpp

I think you forgot to add `static_cast<float>` before using `ceil`
```c++
int reserve_capacity = int(ceil((*root_nodes).size() / num_threads)) * neighs;
```
it should be
```c++
int reserve_capacity = int(ceil( static_cast<float>((*root_nodes).size() / num_threads))) * neighs;
```