Fix policyengine-core so it does not crash Policyengine-US model

Using the following up-to-date versions, Policyengine-US crashes as described at the bottom:

```
% pip list | grep policyengine
policyengine-core 2.21.8
policyengine-us 0.777.2
```

Here is the end of the fatal error traceback:

```
File "..anaconda3/envs/policyengine-us/lib/python3.10/site-packages/
policyengine_core/simulations/simulation.py", line 1349, in _get_macro_cache
self.dataset.file_path.parent
AttributeError: 'str' object has no attribute 'parent'
```

Can this bug be fixed?