Add parameter indexing

[hdoupe]

This PR adds parameter indexing. This is turned on by setting the class attribute indexed to True and by implementing an indexing_rate method. Here's an example that indexes Tax-Calculator's parameters:

import paramtools

class TaxParams(paramtools.Parameters):
    defaults = {
        "schema": {
            "labels": {
                "year": {
                    "type": "int",
                    "validators": {"range": {"min": 2013, "max": 2027}}
                },
                "marital_status": {
                    "type": "str",
                    "validators": {"choice": {"choices": ["single", "joint"]}}
                },
            }
        },
        "standard_deduction": {
            "title": "Standard deduction amount",
            "description": "Amount filing unit can use as a standard deduction.",
            "type": "float",

            # Set indexed to True to extend standard_deduction with the built-in
            # extension logic.
            "indexed": True,

            "value": [
                {"year": 2017, "marital_status": "single", "value": 6350},
                {"year": 2017, "marital_status": "joint", "value": 12700},
                {"year": 2018, "marital_status": "single", "value": 12000},
                {"year": 2018, "marital_status": "joint", "value": 24000},
                {"year": 2026, "marital_status": "single", "value": 7685},
                {"year": 2026, "marital_status": "joint", "value": 15369}],
            "validators": {
                "range": {
                    "min": 0,
                    "max": 9e+99
                }
            }
        },
    }
    array_first = True
    label_to_extend = "year"
    # Activate use of extend_func method.
    uses_extend_func = True
    # inflation rates from Tax-Calculator v2.5.0
    index_rates = {
        2013: 0.0148,
        2014: 0.0159,
        2015: 0.0012,
        2016: 0.0127,
        2017: 0.0187,
        2018: 0.0224,
        2019: 0.0186,
        2020: 0.0233,
        2021: 0.0229,
        2022: 0.0228,
        2023: 0.0221,
        2024: 0.0211,
        2025: 0.0209,
        2026: 0.0211,
        2027: 0.0208,
        2028: 0.021,
        2029: 0.021
    }


params = TaxParams()
params.standard_deduction

# output:
# array([[ 6074.92, 12149.84],
#        [ 6164.83, 12329.66],
#        [ 6262.85, 12525.7 ],
#        [ 6270.37, 12540.73],
#        [ 6350.  , 12700.  ],
#        [12000.  , 24000.  ],
#        [12268.8 , 24537.6 ],
#        [12497.  , 24994.  ],
#        [12788.18, 25576.36],
#        [13081.03, 26162.06],
#        [13379.28, 26758.55],
#        [13674.96, 27349.91],
#        [13963.5 , 27926.99],
#        [ 7685.  , 15369.  ],
#        [ 7847.15, 15693.29]])

TODO:

• Implement way for indexing to be turned on and off for specific parameters.
• Add tests.
• Add docs.

Note: ParamTools will not support updating index rates after the class has been initialized. That is, any updates to the indexing rates should be applied to the rates before this class is initialized.

[hdoupe]

Haven't forgotten about #65--I'll be coming back to it soon. I've been caught up with some other projects.

[hdoupe]

I'm going to leave creating a way to turn indexing on and off for a separate PR. I think the best way to do this is to allow the user to adjust select attributes on a given parameter's JSON object. For example, one could adjust the indexed attribute with this adjustment:

params.adjust({
    "standard_deduction": [{"indexed": false}]
})

Since this has wide-ranging implications for ParamTools, I'm going to tackle it by itself.