Add Policy.tmd_constructor static method

[martinholmer]

Adds convenience to Tax-Calculator Python API when using national or area TMD input data.
There are no changes in tax logic.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (22b99e4) to head (ed04183).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2834   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2617     2617           
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  Hits         2602     2602           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <100.00%> (ø)

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Files with missing lines	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)
taxcalc/policy.py	100.00% <100.00%> (ø)