Making a simple standard for top-level benefit variables: eligibility and amount_if_eligible

[MaxGhenis]

Benefit programs are often structured as two questions:

1. Is the person/unit/family/household eligible?
2. If they are eligible, how much do they get?

Each of these questions is pretty complicated, but this breakdown presents an opportunity to define a standard, by which each benefit program starts with a Variable with a simple formula based only on these two Variables; for example, LIHEAP could be:

class liheap(Variable):

    def formula(spm_unit, period, parameters):
        return where(spm_unit("is_liheap_eligible"), spm_unit("liheap_amount_if_eligible"), 0)

Then the first PR for any new variable would be this along with defining the two sub-variables with no formula.

Thoughts?

[MaxGhenis]

Per a chat with @nikhilwoodruff we'll do this, though with spm_unit("spm_unit_is_liheap_eligible")