Round SNAP after multiplying by expected contribution and monthlyizing

Currently we apply SNAP rounding rules to the annual income:

https://github.com/PolicyEngine/openfisca-us/blob/b5be17ab620a47eb9b60af127fb394526a421f14/openfisca_us/variables/gov/usda/snap/snap_expected_contribution.py#L17-L20

[The law](https://www.law.cornell.edu/uscode/text/7/2017#a), however, rounds the monthly amount after the expected contribution share:
>The value of the [allotment](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1921006204-455106330&term_occur=999&term_src=title:7:chapter:51:section:2017) which[ State ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-80204913-455106346&term_occur=999&term_src=title:7:chapter:51:section:2017)agencies shall be authorized to issue to any[ households ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1256698945-729468644&term_occur=999&term_src=title:7:chapter:51:section:2017)certified as eligible to participate in the [supplemental nutrition assistance program](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1776625920-1223394958&term_occur=999&term_src=title:7:chapter:51:section:2017) shall be equal to the cost to such[ households ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1256698945-729468644&term_occur=999&term_src=title:7:chapter:51:section:2017)of the[ thrifty food plan ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-352542901-455106349&term_occur=999&term_src=)reduced by an amount equal to 30 per centum of the[ household’](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1256698945-729468644&term_occur=999&term_src=)s income, as determined in accordance with section 2014(d) and (e) of this title, rounded to the nearest lower whole dollar: Provided, That for[ households ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1256698945-729468644&term_occur=999&term_src=title:7:chapter:51:section:2017)of one and two persons the minimum[ allotment ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1921006204-455106330&term_occur=999&term_src=)shall be 8 percent of the cost of the[ thrifty food plan ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-352542901-455106349&term_occur=999&term_src=)for a[ household ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1256698945-729468644&term_occur=999&term_src=)containing 1 member, as determined by the[ Secretary ](https://www.law.cornell.edu/definitions/uscode.php?width=840&height=800&iframe=true&def_id=7-USC-1264422296-455106344&term_occur=999&term_src=)under [section 2012 of this title](https://www.law.cornell.edu/uscode/text/7/2012), rounded to the nearest whole dollar increment.

Start in openfisca_us/variables/gov/usda/snap/snap_expected_contribution.py at lines 17-20, then compare the current calculation with the cited SNAP statute. Done means the expected contribution is multiplied and monthlyized before applying the required lower-whole-dollar rounding.

Written by the indexing model from the issue text.