Automatic period adjustment

[nikhilwoodruff]

This PR adds a few features that greatly speed up the process of monthlyising variables in country packages. Specifically:

• Variables defined for years or months automatically cope when input values are set for periods that are contained or contain them. E.g. setting a monthly employment income variable to £30k over the year will divide appropriately.
• Variables automatically are calculated up or down if requested for sub or super periods. E.g. asking for yearly employment income in June will divide down.

I've also widened the default error threshold condition in tests very slightly to 1e-3.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (98576a6) 83.69% compared to head (3079c6e) 83.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   83.69%   83.38%   -0.32%     
==========================================
  Files         182      182              
  Lines        8711     8686      -25     
  Branches     1082     1079       -3     
==========================================
- Hits         7291     7243      -48     
- Misses       1176     1191      +15     
- Partials      244      252       +8     

Files	Coverage Δ
policyengine_core/holders/holder.py	68.59% <100.00%> (-2.84%)	⬇️
policyengine_core/tools/test_runner.py	73.36% <100.00%> (+0.12%)	⬆️
policyengine_core/variables/variable.py	85.76% <100.00%> (-0.52%)	⬇️
tests/core/test_calculate_output.py	100.00% <ø> (ø)
tests/core/test_countries.py	98.46% <ø> (-0.31%)	⬇️
tests/core/test_holders.py	100.00% <ø> (ø)
tests/core/test_reforms.py	98.96% <ø> (-0.05%)	⬇️
policyengine_core/simulations/simulation.py	54.39% <52.38%> (-1.12%)	⬇️

... and 2 files with indirect coverage changes

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