Generalize checking for boolean policy parameter values

[martinholmer]

This pull request implements a request from @hdoupe in PolicyBrain 782.

[codecov-io]

Codecov Report

Merging #1782 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1782   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3028    3028           
======================================
  Hits         3028    3028

Impacted Files	Coverage Δ
taxcalc/policy.py	100% <100%> (ø)	⬆️

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[hdoupe]

Thanks @martinholmer. I think this should work. I'll need to upgrade to Tax-Calculator 0.14.2 before I can test this. I'll have some feed back for you before the end of the day.

[hdoupe]

@martinholmer This resolves the issue described in PolicyBrain #780. I implemented a temporary fix in this commit. This fix will be removed once the next version of Tax-Calculator is released.

The fix isn't necessarily bad, but it does introduce unneeded complexity. Further, I think it makes PolicyBrain a little too opinionated. Past experience suggests that parameter checking and conversion at this level should be left to taxcalc.

Thanks for quickly addressing this issue.