Consolidate all the private validate logic in the Parameters class

[martinholmer]

This pull request consolidates all the parameter _validate_names_types and _validate_values logic in the Parameters class. This change reduces code duplication and makes the abstract Parameters class a better candidate for using to build policy parameter and/or assumption parameter classes in other models that are already importing the taxcalc package. This pull request also removes from the Parameters class code all hardcoded names of wage-indexed parameters. And it moves the code that reads the DEFAULTS JSON file to create a _vals dictionary into the Parameters class constructor, where it should have been in the original design of the Parameters class.

[codecov]

Codecov Report

Merging #2281 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2281   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          12      12           
  Lines        2997    2930   -67     
======================================
- Hits         2997    2930   -67

Impacted Files	Coverage Δ
taxcalc/growdiff.py	100% <100%> (ø)	⬆️
taxcalc/parameters.py	100% <100%> (ø)	⬆️
taxcalc/consumption.py	100% <100%> (ø)	⬆️
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 c093e59...f7742fd. Read the comment docs.