calc_spending_by_month: Calculate New Enrollee Part D Spending by Month
In evanjflack/cfo.behavioral: Behavioral Hazard and Patient Decision Making

calc_spend_by_month() is a function that calculates both the total/OOP spending by calendar month, and cumulative amounts for each within a year.

1	calc_spending_by_month(DT, DT_id)

`DT`	data.table with the columns bene_id, year (relative to enrollment year, starting at 1), month, cost, and oop_cost
`DT_id`	data.table with the column bene_id

data.table with 7 columns:

`bene_id`
`year`
`month`
`cost`	Total spending in month
`cum_cost`	Total spending though month in year
`oop_cost`	OOP spending in month
`cum_oop_cost`	Total OOP spending through month in year