Generalize policy parameter suffix logic to handle non-default baseline Growdiff

[martinholmer]

This pull request generalizes the policy parameter suffix logic added in pull request #1505.
The old logic in #1505 works fine as long as the default price inflation and wage growth rates are being assumed (as they are when specifying a Tax-Calculator simulation using the TaxBrain GUI input page). However, when using the TaxBrain file upload page or when using Tax-Calculator on a local computer, the old #1505 logic was incorrect when using the growdiff_baseline dictionary to specify non-default price inflation or wage growth rates. This pull request fixes that problem. Other than this change, there are no changes to tax-calculating logic or tax results.

@MattHJensen @Amy-Xu @andersonfrailey @hdoupe @GoFroggyRun @codykallen

[codecov-io]

Codecov Report

Merging #1520 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1520   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        2557    2565    +8     
======================================
+ Hits         2557    2565    +8

Impacted Files	Coverage Δ
taxcalc/policy.py	100% <100%> (ø)	⬆️
taxcalc/growdiff.py	100% <100%> (ø)	⬆️
taxcalc/calculate.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 e981d56...01e49ce. Read the comment docs.