Use pregnancies instead of is_pregnant for Medicaid

[MaxGhenis]

This CMS document states:

For Medicaid eligibility, if a pregnant woman is seeking an eligibility determination for herself, she is counted as one person plus the number of children expected to deliver. However, if a pregnant woman is in the household of someone who is seeking a Medicaid eligibility determination, states can opt to count her as one person, two people, or one person plus the number of children expected.

We currently only count the pregnant woman as one additional person in Medicaid, since we only have the is_pregnant bool. We should instead add a pregnancies variable (or perhaps some other term) to indicate the number of children expected to deliver. This may apply to other programs as well.