Strengthen type checking of reform parameters

[martinholmer]

This pull request attempts to resolve the bug report by @hdoupe in issue #1956.
It does so in a way that strengthens the type checking of all parameters (boolean, integer and float) in a reform dictionary passed to the Policy class implement_reform method.

[codecov-io]

Codecov Report

Merging #1960 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1960   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3748    3756    +8     
======================================
+ Hits         3748    3756    +8

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2b4a3ea...6fb3929. Read the comment docs.

[hdoupe]

@martinholmer this looks good to me.

[martinholmer]

@hdoupe, Thanks for the review. Merging now.