[FIX] make function call on the right model

easy_my_coop_loan_account/models/interest_line.py

@@ -100,7 +100,7 @@ class LoanInterestLine(models.Model):
                 # compute the prorata interest for the fiscal year
                 prorata_date = line.due_date - relativedelta(years=-1)
                 diff_days = (prorata_date - date).days
-                days = line.loan_issue_id.get_number_of_days(date.year)
+                days = line.issue_line.get_number_of_days(date.year)
 
                 previous_interest = line.accrued_interest - line.interest
                 prorata_interest = line.interest * (diff_days / days)