Clarify datatype of parameters in JSON files

[martinholmer]

This pull request tries to make the JSON parameter file values of integer_value and boolean_value less confusing.

Now, a boolean parameter has integer_value false and boolean_value true.
And an integer parameter has integer_value true and boolean_value false.
And a float parameter has integer_value false and boolean_value false.

The Tax-Calculator tests have been changed to enforce this new approach and the source code has been modified to handle the new pattern of values.

These changes do imply a change in the user documentation, but they cause no changes in test results or calculated tax results.

Thanks to @hdoupe for pointing out the need for these changes in his extensive work on pull request #1952.

[codecov-io]

Codecov Report

Merging #1970 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1970   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3756    3762    +6     
======================================
+ Hits         3756    3762    +6

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

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