Parametrize 2.5% cap on AMT medical expenses deduction as share of AGI

[MaxGhenis]

Currently hard-coded at 2.5% in the AMT formula:

Tax-Calculator/taxcalc/calcfunctions.py

Lines 1976 to 1979 in cf452aa

	if standard == 0.0:
	c62100 = (c00100 - e00700 - c04470 +
	max(0., min(c17000, 0.025 * c00100)) +
	c18300 + c20800 - c21040)

This was line 2 in form 6251 until 2016:

Medical and dental. If you or your spouse was 65 or older, enter the smaller of Schedule A (Form 1040), line 4, or 2.5% (0.025) of Form 1040, line 38. If zero or less, enter -0-

TCJA removed it, so the number no longer appears as of the 2017 form 6251.