Add weights_scale attribute to the Records and Data classes

[martinholmer]

Provides flexibility in converting weights in the *_weights.csv.gz files into s006 values.

This flexibility is essential when using tax-microdata area weights as described in TMD PR #264.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (cfca1d5) to head (163d040).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2828   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2615     2616    +1     
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+ Hits         2600     2601    +1     
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)
taxcalc/data.py	99.25% <100.00%> (+<0.01%)	⬆️
taxcalc/records.py	100.00% <100.00%> (ø)
taxcalc/taxcalcio.py	100.00% <100.00%> (ø)