Parametrize 25% multiplier in `line61` variable of AMT function

[MaxGhenis]

Currently hard-coded:

Tax-Calculator/taxcalc/calcfunctions.py

Line 2033 in cf452aa

line61 = 0.25 * max(0., line30 - line41 - line56 - line57 - linex2)

As an aside, this is line 37 in the current form 6251, not line 61, due to TCJA shortening the AMT form.

[jdebacker]

@MaxGhenis It looks like PolicyEngine has this hardcoded as well. Trying to think if a good name if one wants to parameterize this...

[MaxGhenis]

FWIW ChatGPT thought capital_gain_excess_tax_rate would be an appropriate name, at least in the PolicyEngine naming convention: https://chat.openai.com/share/35162ac1-0ac3-4caa-91b8-4ee34f7879d5