update tutorial using get_example_filepath

It now returns a Path object so this code is not valid anymore:
```python
csv_dir = get_example_filepath('examples')
population = read_csv(csv_dir + '/population.csv')
```
and should be changed to:
```python
csv_dir = get_example_filepath('examples')
population = read_csv(csv_dir / 'population.csv')
```
There might be some other usage outside of the tutorial.