Move logic for reading JSON parameter revisions to Parameter class

[martinholmer]

This pull request simplifies the process of specifying and reading JSON reform files by moving the logic for reading JSON parameter revisions from the Calculator class to the Parameter class, which is a more sensible place for it. The simplification in specifying JSON policy reform files is that the top-level "policy" key is no longer mandatory. Also, when reading just a JSON policy reform file, the reading of it has been made easier by the addition of the Policy.read_json_reform method.

So, before this pull request a JSON reform file had to be specified this way:

{
    "policy": {
        "II_em": {"2022": 1000}
    }
}

but now it can be simplified to this:

{
    "II_em": {"2022": 1000}
}

Assume that the above JSON (either version) is in a file whose name is contained in the json string variable.

Before this pull request that JSON reform file had to be used this way:

pol = Policy()
param_dict = Calculator.read_json_param_objects(json, None)
pol.implement_reform(param_dict['policy'])

but now it is easier:

pol = Policy()
pol.implement_reform(Policy.read_json_reform(json))

Note that the old specification of JSON reform files still works and the old way of reading a JSON reform file is still available. It is just that now there are easier ways to do these things than by using the legacy code.

[codecov]

Codecov Report

Merging #2312 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2312   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          12      12           
  Lines        2839    2809   -30     
======================================
- Hits         2839    2809   -30

Impacted Files	Coverage Δ
taxcalc/consumption.py	100% <100%> (ø)	⬆️
taxcalc/parameters.py	100% <100%> (ø)	⬆️
taxcalc/growdiff.py	100% <100%> (ø)	⬆️
taxcalc/calculator.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 2084369...7653c85. Read the comment docs.