find_rate: Find the rate for a loan given the discount factors
Description
Thru a root finding process, this function finds the rate that corresponds to a
given set of discount factors, as for the loan to have the same present value discounted with the
discount factors or with that constant rate
Usage
find_rate(m, d, loan_type, interval = c(1e-06, 2), tol = 1e-08)