Add basic EconomicImpact class structures

[nikhilwoodruff]

Tentatively, unless we have better ideas, we're envisioning:

from policyengine import EconomicImpact

impact = EconomicImpact(reform={"gov.basic_rate": 0.25}, country="uk")
impact.calculate("inequality/gini")

>>> {"baseline": 0.3, "reform": 0.35, "change": 0.05}

Also, maybe:

from policyengine import Economy

baseline = Economy(country="uk")

baseline.calculate("inequality/gini")

>>> 0.35

EconomicImpact(baseline=baseline, reform=reform_economy).calculate("inequality/gini")

>>> {"baseline": 0.3, "reform": 0.35, "change": 0.05}