Microsimulation fixes

[nikhilwoodruff]

This PR:

• Improves, documents and adds tests for code in get_micro_data.py (mainly the household MTR function)
• Should fix @jpycroft's last issue in Error running the example script #31 (the synthetic dataset isn't automatically downloading)
• Fixes the example script to run by default

[nikhilwoodruff]

Here are the outputs produced by examples/plot.py now (the tax functions in 3D plots):

[codecov]

Codecov Report

Merging #35 (6a148c1) into main (857532c) will increase coverage by 17.26%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main      #35       +/-   ##
===========================================
+ Coverage   45.85%   63.12%   +17.26%     
===========================================
  Files           6        4        -2     
  Lines         205      141       -64     
===========================================
- Hits           94       89        -5     
+ Misses        111       52       -59     

Flag	Coverage Δ
unittests	63.12% <100.00%> (+17.26%)	⬆️

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Impacted Files	Coverage Δ
oguk/get_micro_data.py	70.96% <100.00%> (-5.35%)	⬇️
oguk/tests/test_get_micro_data.py	87.50% <100.00%> (+3.62%)	⬆️

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[jdebacker]

@nikhilwoodruff Thanks for this PR. LGTM. Merging.