Add flat file export

[nikhilwoodruff]

Fixes #236

[nikhilwoodruff]

Tested on the UK system

[codecov]

Codecov Report

Attention: Patch coverage is 3.03030% with 32 lines in your changes missing coverage. Please review.

Project coverage is 79.53%. Comparing base (d0ee394) to head (82159d3).
Report is 9 commits behind head on master.

Files	Patch %	Lines
policyengine_core/simulations/simulation.py	3.44%	28 Missing ⚠️
policyengine_core/data/dataset.py	0.00%	2 Missing ⚠️
policyengine_core/reforms/reform.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
- Coverage   81.98%   79.53%   -2.46%     
==========================================
  Files         189      191       +2     
  Lines        9532    10005     +473     
  Branches     1205     1306     +101     
==========================================
+ Hits         7815     7957     +142     
- Misses       1436     1767     +331     
  Partials      281      281              

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