Rename Calculator.param method as Calculator.policy_param

[martinholmer]

This pull request renames the param method of the Calculator class to policy_param in order to clarify differences between the various types of parameters (Consumption, Behavior, Policy, and more to come).

This is a public API change for users of Tax-Calculator who call the old Calculator.param method in their applications. The Tax-Calculator library has been update internally to use the new Calculator.policy_param method, but other applications will have to be updated by their developers.

[codecov-io]

Codecov Report

Merging #1915 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1915   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3319    3319           
======================================
  Hits         3319    3319

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

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