Not implemented. Compute the payment portion for loan interest.
Parameters :  rate : scalar or array_like of shape(M, )
per : scalar or array_like of shape(M, )
nper : scalar or array_like of shape(M, )
pv : scalar or array_like of shape(M, )
fv : scalar or array_like of shape(M, ), optional
when : {{‘begin’, 1}, {‘end’, 0}}, {string, int}, optional


Returns :  out : ndarray

Notes
The total payment is made up of payment against principal plus interest.
pmt = ppmt + ipmt