Handle larger integer parameter values

[martinholmer]

This pull request simply increases the size of each element in integer arrays holding parameters from np.int8 to np.int16.

[codecov]

Codecov Report

Merging #2269 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2269   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          12      12           
  Lines        2997    2997           
======================================
  Hits         2997    2997

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

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