Add examples for both numerical/boolean formulas

[bonjourmauko]

Newcomers may want to use OpenFisca for eligibility/condition checks.

This is supported, but unclear.

It'd be great to have at least one example of a numerical formula, and another of a boolean one.

Proposed example for the latter:

class basic_income_age_condition(Variable):
    value_type = bool
    entity = Person
    definition_period = MONTH
    label = "Age eligibility condition for basic income"
    reference = "https://law.gov.example/basic_income/eligibility/age"

    def formula(persons, period, parameters):
        input_age = persons("age", period)
        threshold = parameters(period).general.age_of_majority
        return input_age >= threshold