Provide guidance on parameters and variables with the same name

For example, consider the case of identifying whether a household is in poverty, where poverty line is indexed by household size. The formula could include Python variables that both represent the poverty line parameter (which is indexed by household size) and the household's poverty line based on their specific household size. I see three ways to address this:

A: Add a qualifier to the entity's value, e.g. `household_`

class is_in_poverty(Variable):
    def formula(household, period, parameters):
        poverty_line = parameters(period).poverty.poverty_line
        household_size = household.nb_persons()
        household_poverty_line = poverty_line[household_size]
        return household("household_income") < household_poverty_line

B: Add a qualifier to the parameter, e.g. `param_` or `p_`

class is_in_poverty(Variable):
    def formula(household, period, parameters):
        param_poverty_line = parameters(period).poverty.poverty_line
        household_size = household.nb_persons()
        poverty_line = poverty_line[household_size]
        return household("household_income") < poverty_line

C: Add a separate OpenFisca variable for any parameter indexed by household features

class household_poverty_line(Variable):
    def formula(household, period, parameters):
        poverty_line = parameters(period).poverty.poverty_line
        household_size = household.nb_persons()
        return poverty_line[household_size]

class is_in_poverty(Variable):
    def formula(household, period, parameters):
        return household("household_income") < household("household_poverty_line")

Option C creates a lot of variables, but it seems cleanest to me. I think it would avoid the issue of potentially overlapping variables to represent parameters and Variables, but there might be some cases I'm not thinking of.

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