Add Simulation.subsample

[nikhilwoodruff]

Fixes #278

[codecov]

Codecov Report

Attention: Patch coverage is 8.57143% with 32 lines in your changes missing coverage. Please review.

Project coverage is 79.51%. Comparing base (cf53c6d) to head (8bfcd1a).

Files with missing lines	Patch %	Lines
policyengine_core/simulations/simulation.py	8.57%	31 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
- Coverage   79.73%   79.51%   -0.23%     
==========================================
  Files         192      192              
  Lines       10057    10088      +31     
  Branches     1311     1317       +6     
==========================================
+ Hits         8019     8021       +2     
- Misses       1753     1781      +28     
- Partials      285      286       +1     

Flag	Coverage Δ
	79.51% <8.57%> (?)

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[nikhilwoodruff]

The Windows failure is a documentation failure which isn't critical, so merging.