Rename classes as GrowFactors and GrowDiff

[martinholmer]

This pull request adopts the following PEP8 prescriptive naming convention for class names:

Class names should normally use the CapWords convention.

So, the Growfactors class has been renamed the GrowFactors class.
And the Growdiff class has been renamed the GrowDiff class.

Any Python scripts that use these two classes will need to be revised.

Also, the constructor dictionary argument in several classes derived from the ParametersBase class has been removed as was done for the Behavior class in pull request #1952.

[codecov-io]

Codecov Report

Merging #1996 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1996   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3782    3782           
======================================
  Hits         3782    3782

Impacted Files	Coverage Δ
taxcalc/taxcalcio.py	100% <100%> (ø)	⬆️
taxcalc/consumption.py	100% <100%> (ø)	⬆️
taxcalc/behavior.py	100% <100%> (ø)	⬆️
taxcalc/growdiff.py	100% <100%> (ø)	⬆️
taxcalc/records.py	100% <100%> (ø)	⬆️
taxcalc/policy.py	100% <100%> (ø)	⬆️
taxcalc/growfactors.py	100% <100%> (ø)	⬆️
taxcalc/calculate.py	100% <100%> (ø)	⬆️

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