Parameterize the `2500` value in `AGI` formula

[MaxGhenis]

The hard-coded 2500 in the AGI formula here looks like a phase-out start:

Tax-Calculator/taxcalc/calcfunctions.py

Lines 712 to 723 in cf452aa

	# phase-out personal exemption amount
	if exact == 1: # exact calculation as on tax forms
	line5 = max(0., c00100 - II_em_ps[MARS - 1])
	line6 = math.ceil(line5 / (2500. / sep))
	line7 = II_prt * line6
	c04600 = max(0., pre_c04600 * (1. - line7))
	else: # smoothed calculation needed for sensible mtr calculation
	dispc_numer = II_prt * (c00100 - II_em_ps[MARS - 1])
	dispc_denom = 2500. / sep
	dispc = min(1., max(0., dispc_numer / dispc_denom))
	c04600 = pre_c04600 * (1. - dispc)
	return (c00100, pre_c04600, c04600)

[duncanhobbs]

It looks like the 2500 is part of the personal exemption phaseout.

From section 151 of the internal revenue code:

For each $2,500 (or fraction thereof) by which the taxpayer’s adjusted gross income for the taxable year exceeds the applicable amount, the personal exemption is reduced by 2 percent.