Eliminate shortcomings in calculation of the ACA premium tax credit

The 0.513.0 version of PolicyEngine-US has the capability to compute the Affordable Care Act (ACA) premium tax credit (PTC) for a tax unit. But the current capability has a number of shortcomings that should be eliminated. Here are some of those shortcomings:

* Not all counties are included in the four-hundred-some ACA geographical rating areas (GRA). Examples include Harris County TX (Houston) and Marion County IN (Indianapolis).

* ACA PTC eligibility rules have not been updated to reflect recent legislation.

* Values for the second-lowest ACA silver-plan cost (SLSPC) are not available for NY and VT in 2022 and are not available for any state in 2023.

* The value of the ACA modified AGI to federal poverty line variable is incorrect in that it is calculated using the current-year poverty line instead of the prior-year poverty line, and hence, all the PTC amounts are incorrect.

* The time required to compute a PTC amount is considerable because of inefficiencies in the handling of GRA and SLSPC parameter files and in the coding of ACA logic.

Eliminating these shortcomings will be done in two steps:

(1) the `aca-ca` branch, which implements an alternative ACA PTC capability at the federal level and for California, will be submitted as a pull request; and

(2) an `aca-rest` branch, which implements the alternative ACA PTC capability for the other 49 states and for DC, will be submitted as a pull request.

After taking these two steps the old ACA PTC calculation capability will still be intact and there will also be a new ACA PTC calculation capability. At that stage, we will have the option of changing the list of federal refundable credits to include the new `aca_ptc` variable instead of the old `premium_tax_credit` variable, and of removing the old ACA PTC calculation capability.