Monthlyisation fix

[nikhilwoodruff]

Fixes #164

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.55%. Comparing base (94b78ef) to head (e26ee89).

Files	Patch %	Lines
policyengine_core/data/dataset.py	50.00%	0 Missing and 1 partial ⚠️
policyengine_core/simulations/simulation.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #165   +/-   ##
=======================================
  Coverage   82.55%   82.55%           
=======================================
  Files         185      185           
  Lines        8998     8996    -2     
  Branches     1128     1127    -1     
=======================================
- Hits         7428     7427    -1     
- Misses       1301     1302    +1     
+ Partials      269      267    -2     

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