Fix CDCC phase-out logic

[martinholmer]

Since March 2021 when PR #2573 was merged into the master branch, the CDCC phase-out calculations have not been correct. The problem stems from the introduction in PR #2573 of the CDCC_prt parameter, which is defined as the ratio of the rate phase-out per incremental step of AGI above the CDCC_ps AGI level (and, only in 2021, the CDCC_ps2) and the size of the incremental steps of excess AGI. Because the phase-out rate per step is 0.01 and the size of each step is 2000, the CDCC_prt parameter value was set to 0.01 / 2000, which is 5e-6.

The problem is that in the exact calculations, the number of phase-out steps is a stair-step function, with fractional steps rounded up to the next highest whole number of steps. That calculation was unable to be done correctly when there was only the compound CDCC_prt parameter.

The solution is to replace the CDCC_prt parameter with two new parameters: the CDCC_po_step_size parameter (which has a value of 2000) and the CDCC_po_rate_per_step parameter (which has a value of 0.01). Then the code will be able to apply correctly the ceiling function to the number of phase-out steps.

Here is a simple 2022 test case that illustrates the problem:

% cat case.csv      
RECID,MARS,age_head,age_spouse,n21,n1820,nu18,EIC,n24,nu13,f2441,nu06,e00200p,e00200s,e00200,e32800
29031,4,36,0,1,0,1,1,1,1,1,0,41750,0,41750,8000

samples% tc case.csv 2022 --exact --dump --dvars tc.ovars 
Tax-Calculator startup did not extrapolate your data.

samples% cat case-22-\#-\#-\#.csv
FLPDYR,RECID,c00100,c02500,c04800,c05800,c07180,c09200,ctc_refundable,ctc_total,eitc,iitax,payrolltax,ptax_amc,recovery_rebate_credit,refund
2022,29031,41750.00,0.00,22350.00,2389.00,600.00,0.00,211.00,2000.00,278.12,-489.12,6387.75,0.00,0.00,489.12

Notice that AGI is $41,750 and the calculated value of CDCC (c07180) is $600, which is incorrect; it should be $630. Also note that (41750 - 15000) / 2000 = 13.375, which rounds up to 14 steps. And that 0.35 - 14 * 0.01 = 0.21 (not 0.20). The difference between 0.21 and 0.20 causes the $30 CDCC difference.