rate: interest rate per period
nper: number of periods
pv: present value
start_period: first period to accumulate for
end_period: last period to accumulate for
type: payment type
CUMPRINC calculates the cumulative principal paid on a loan from start_period to end_period.
If type is 0, the default, payment is at the end of each period. If type is 1, payment is at the beginning of each period.