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

[martinholmer]

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?