Add switch to phase basic income in

[MaxGhenis]

Should default to false. Currently we phase it in at 100% rate:

• https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/variables/contrib/ubi_center/basic_income/basic_income_phase_in.py
• https://policyengine.org/us/policy?reform=2&focus=gov.contrib.ubi_center.basic_income.phase_in.rate

Example:

Also add a test to ensure under default conditions someone with no earnings gets the full basic income.

[MaxGhenis]

Verified fixed